Relatively few types are ever used as map keys,
so tracking this separately is a net win.

Passes toolstash -cmp.

name       old alloc/op     new alloc/op     delta
Template       55.9MB ± 0%      55.5MB ± 0%  -0.71%        (p=0.000 n=10+10)
Unicode        37.8MB ± 0%      37.7MB ± 0%  -0.27%        (p=0.000 n=10+10)
GoTypes         180MB ± 0%       179MB ± 0%  -0.52%         (p=0.000 n=7+10)
Compiler        806MB ± 0%       803MB ± 0%  -0.41%        (p=0.000 n=10+10)

CPU and number of allocs are unchanged.

Change-Id: I6d60d74a4866995a231dfed3dd5792d75d904292
Reviewed-on: https://go-review.googlesource.com/21622Reviewed-by: Matthew Dempsky <mdempsky@google.com>