cmd/go: implement 'internal' convention

See golang.org/s/go14internal for design.

LGTM=r
R=r, adg
CC=golang-codereviews
https://codereview.appspot.com/120600043
runtime: use better hash for floating point inputs

Hashing on the bytes instead of the words does
a (much) better job of using all the bits, so that
maps of floats have linear performance.

LGTM=khr
R=golang-codereviews, khr
CC=adonovan, golang-codereviews
https://codereview.appspot.com/126720044