Speedup and cleanup vom decoder.
Speedup and cleanup vom decoder.

This brings the vom Decoder implementation into a reasonable
state, performance-wise.  The main changes are to:

1) Bytes handling is improved, both for []byte lists and [n]byte
arrays, and regardless of named / unnamed / generated / reflect
variants.  Note that the implementation chooses to not re-use
existing byte slices when decoding, which costs ~10% in our
benchmarks, but is safer for developers.

2) Scalar collections now have special Decoder ReadValue* and
NextEntryValue* methods.  This simplifies the generated code, and
also lets us implement some important optimizations, which speed
up array, list, set and map.

Other than similar changes on the Encoder, the largest remaining
item for vom speedup / cleanup is to re-implement the vdl type
registration mechanism, which will let us use David Crawshaw's
cool ideas for smaller binary sizes, and also for a faster /
simpler implementation.

There are lots of files here.  The important ones to look at are
the non *.vdl.go files under:
  v.io/v23/vdl
  v.io/v23/vom
  v.io/v23/verror
  v.io/x/ref/lib/vdl/codegen/golang

Here are the decoding benchmarks.  I'm not worrying too much
about the ones that got worse, since the absolute magnitude is
relatively small.

benchmark                          old ns/op  new ns/op    delta
Vom___Decode_____XNumber               1477       1318   -10.77%
Vom___DecodeMany_XNumber                536        459   -14.37%
Vom___Decode_____VNumber               1045        898   -14.07%
Vom___DecodeMany_VNumber                260        256    -1.54%
Vom___Decode_____XStringSmall          1627       1437   -11.68%
Vom___DecodeMany_XStringSmall           681        617    -9.40%
Vom___Decode_____VStringSmall          1207       1012   -16.16%
Vom___DecodeMany_VStringSmall           405        343   -15.31%
Vom___Decode_____XStringLarge         33714      33658    -0.17%
Vom___DecodeMany_XStringLarge         33325      32451    -2.62%
Vom___Decode_____VStringLarge         33284      32038    -3.74%
Vom___DecodeMany_VStringLarge         31765      31426    -1.07%
Vom___Decode_____VEnum                 1123        910   -18.97%
Vom___DecodeMany_VEnum                  298        271    -9.06%
Vom___Decode_____XByteListSmall        1950       1592   -18.36%
Vom___DecodeMany_XByteListSmall         931        802   -13.86%
Vom___Decode_____VByteListSmall        1225       1052   -14.12%
Vom___DecodeMany_VByteListSmall         426        386    -9.39%
Vom___Decode_____XByteListLarge       20721      19814    -4.38%
Vom___DecodeMany_XByteListLarge       19334      18850    -2.50%
Vom___Decode_____VByteListLarge       19364      18905    -2.37%
Vom___DecodeMany_VByteListLarge       17947      17987    +0.22%
Vom___Decode_____XByteArray            1758       1568   -10.81%
Vom___DecodeMany_XByteArray             768        733    -4.56%
Vom___Decode_____VByteArray            1125        945   -16.00%
Vom___DecodeMany_VByteArray             358        332    -7.26%
Vom___Decode_____XArray                2553       1840   -27.93%
Vom___DecodeMany_XArray                1234        733   -40.60%
Vom___Decode_____VArray                1515       1199   -20.86%
Vom___DecodeMany_VArray                 502        409   -18.53%
Vom___Decode_____XListSmall            3719       2178   -41.44%
Vom___DecodeMany_XListSmall            2344       1088   -53.58%
Vom___Decode_____VListSmall            1608       1255   -21.95%
Vom___DecodeMany_VListSmall             589        465   -21.05%
Vom___Decode_____XListLarge        28288185    3706132   -86.90%
Vom___DecodeMany_XListLarge        28454774    3718255   -86.93%
Vom___Decode_____VListLarge         5264233    2425440   -53.93%
Vom___DecodeMany_VListLarge         6324235    2424557   -61.66%
Vom___Decode_____XListAnySmall         5459       4371   -19.93%
Vom___DecodeMany_XListAnySmall         3895       2918   -25.08%
Vom___Decode_____VListAnySmall         2898       2666    -8.01%
Vom___DecodeMany_VListAnySmall         1701       1507   -11.41%
Vom___Decode_____XListAnyLarge     56238352   38693410   -31.20%
Vom___DecodeMany_XListAnyLarge     56389732   37919725   -32.75%
Vom___Decode_____VListAnyLarge     27310371   25149237    -7.91%
Vom___DecodeMany_VListAnyLarge     26423353   24243553    -8.25%
Vom___Decode_____VSet                  2291       1970   -14.01%
Vom___DecodeMany_VSet                  1227       1088   -11.33%
Vom___Decode_____XMap                  5278       4169   -21.01%
Vom___DecodeMany_XMap                  4734       3048   -35.61%
Vom___Decode_____VMap                  2699       2144   -20.56%
Vom___DecodeMany_VMap                  1527       1209   -20.83%
Vom___Decode_____XSmallStruct          3860       3506    -9.17%
Vom___DecodeMany_XSmallStruct          2451       2353    -4.00%
Vom___Decode_____VSmallStruct          1878       1627   -13.37%
Vom___DecodeMany_VSmallStruct           823        674   -18.10%
Vom___Decode_____XLargeStruct         33192      30363    -8.52%
Vom___DecodeMany_XLargeStruct         31344      28640    -8.63%
Vom___Decode_____VLargeStruct          8687       7311   -15.84%
Vom___DecodeMany_VLargeStruct          7038       6028   -14.35%
Vom___Decode_____XLargeStructZero      2121       2069    -2.45%
Vom___DecodeMany_XLargeStructZero       758        726    -4.22%
Vom___Decode_____VLargeStructZero      1319       1338    +1.44%
Vom___DecodeMany_VLargeStructZero       293        298    +1.71%
Vom___Decode_____VSmallUnion           2706       2516    -7.02%
Vom___DecodeMany_VSmallUnion           1237       1241    +0.32%
Vom___Decode_____Time                  3078       2812    -8.64%
Vom___DecodeMany_Time                  1800       1672    -7.11%
Vom___Decode_____Blessings            57178      56034    -2.00%
Vom___DecodeMany_Blessings            54852      53810    -1.90%
Vom___Decode_____RPCRequestZero        1765       1767    +0.11%
Vom___DecodeMany_RPCRequestZero         309        327    +5.83%
Vom___Decode_____RPCRequestFull       58691      57640    -1.79%
Vom___DecodeMany_RPCRequestFull       55820      54865    -1.71%
Vom___Decode_____RPCResponseZero       1804       1803    -0.06%
Vom___DecodeMany_RPCResponseZero        338        333    -1.48%
Vom___Decode_____RPCResponseFull      43685      42907    -1.78%
Vom___DecodeMany_RPCResponseFull      39505      39474    -0.08%

MultiPart: 4/5
Change-Id: I675d8c572c3242a56f90f936cc05411c1e180e95

vanadium/physical-lock