Make substr() a lot faster, make substr-rw() work
Make substr() a lot faster, make substr-rw() work

Highlights:
- substr("foo",1)       2.5x faster
- substr("foo",1,2)     6x faster
- substr("foo",*-2)     1.5x faster
- substr("foo",0,*-2)   1.5x faster
- substr-rw() now accepts the same range of parameters as substr()

substr() is now a multi, with 4 candidates.

The same was attempted with substr-rw(), but apparently the Proxy overhead
drowns out any efficiency gains.  And the one sub was fixed some todo tests
for substr-rw, so I left it at one and only candidate for substr-rw().

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