builtin: add documentation for builtins

builtin: add documentation for builtins
Do this by adding a fake package called builtin. At the moment, godoc will
not present this documentation because the function names are all lower case,
but there are plans to address this.
The print and println functions are undocumented here. I could be talked
into doing them but I'd rather not promote their use.

R=dsymonds, n13m3y3r, r, gri, rsc
CC=golang-dev
https://golang.org/cl/4907041