gc: add delete(m, x) but leave in m[x] = 0, false.

gc: add delete(m, x) but leave in m[x] = 0, false.

The old m[x] = 0, false syntax will be deleted
in a month or so, once people have had time to
change their code (there is a gofix in a separate CL).

R=ken2
CC=golang-dev
https://golang.org/cl/5265048