Based on the observation that a great number of the types that
are copied or compared in interfaces, maps, and channels are
word-sized, this uses specialized copy and equality functions
for them that use a word instead of 4 or 8 bytes. Seems to yield
0-6% improvements in performance in the benchmarks I've run.
For example, with the regexp benchmarks:

Before:
regexp.BenchmarkLiteral   500000       3.26 µs/op
regexp.BenchmarkNotLiteral    100000      13.67 µs/op
regexp.BenchmarkMatchClass    100000      18.72 µs/op
regexp.BenchmarkMatchClass_InRange    100000      20.04 µs/op
regexp.BenchmarkReplaceAll    100000      27.85 µs/op

After:
regexp.BenchmarkLiteral   500000       3.11 µs/op
regexp.BenchmarkNotLiteral    200000      13.29 µs/op
regexp.BenchmarkMatchClass    100000      17.65 µs/op
regexp.BenchmarkMatchClass_InRange    100000      18.49 µs/op
regexp.BenchmarkReplaceAll    100000      26.34 µs/op

R=rsc
CC=golang-dev
https://golang.org/cl/1967047