cmd/compile: move hiter, hmap, and scase definitions into builtin.go

cmd/compile: move hiter, hmap, and scase definitions into builtin.go

Also eliminates per-maptype hiter and hmap types, since they're not
really needed anyway.  Update packages reflect and runtime
accordingly.

Reduces golang.org/x/tools/cmd/godoc's text segment by ~170kB:

   text	   data	    bss	    dec	    hex	filename
13085702	 140640	 151520	13377862	 cc2146	godoc.before
12915382	 140640	 151520	13207542	 c987f6	godoc.after

Updates #6853.

Change-Id: I948b2bc1f22d477c1756204996b4e3e1fb568d81
Reviewed-on: https://go-review.googlesource.com/16610
Reviewed-by: Keith Randall <khr@golang.org>