This changes the map lookup behavior for string maps with 2-8 keys.

There was already previously a fastpath for 0 items and 1 item.

Now, if a string-keyed map has <= 8 items, first check all the
keys for length first. If only one has the right length, then
just check it for equality and avoid hashing altogether. Once
the map has more than 8 items, always hash like normal.

I don't know why some of the other non-string map benchmarks
got faster. This was with benchtime=2s, multiple times. I haven't
anything else getting slower, though.

benchmark                             old ns/op    new ns/op    delta
BenchmarkHashStringSpeed                     37           34   -8.20%
BenchmarkHashInt32Speed                      32           29  -10.67%
BenchmarkHashInt64Speed                      31           27  -12.82%
BenchmarkHashStringArraySpeed               105           99   -5.43%
BenchmarkMegMap                          274206       255153   -6.95%
BenchmarkMegOneMap                           27           23  -14.80%
BenchmarkMegEqMap                        148332       116089  -21.74%
BenchmarkMegEmptyMap                          4            3  -12.72%
BenchmarkSmallStrMap                         22           22   -0.89%
BenchmarkMapStringKeysEight_32               42           23  -43.71%
BenchmarkMapStringKeysEight_64               55           23  -56.96%
BenchmarkMapStringKeysEight_1M           279688           24  -99.99%
BenchmarkIntMap                              16           15  -10.18%
BenchmarkRepeatedLookupStrMapKey32           40           37   -8.15%
BenchmarkRepeatedLookupStrMapKey1M       287918       272980   -5.19%
BenchmarkNewEmptyMap                        156          130  -16.67%

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/7641057