myers: fix stupid amounts of allocation and copying
myers: fix stupid amounts of allocation and copying

that was shockingly lazy code to have written, TODO or not TODO.
fix it.

name                               old time/op    new time/op    delta
Golden/testdata/rewriteAMD64.go-8     23.9s ±42%      0.5s ± 2%  -97.98%  (p=0.000 n=10+9)

name                               old alloc/op   new alloc/op   delta
Golden/testdata/rewriteAMD64.go-8    19.5GB ± 0%     0.8GB ± 0%  -96.02%  (p=0.000 n=9+10)

name                               old allocs/op  new allocs/op  delta
Golden/testdata/rewriteAMD64.go-8      150k ± 0%      150k ± 0%   -0.00%  (p=0.000 n=10+10)

pkg/diff