fmt: format 0b, 0o prefixes in %#b and %O

This CL modifies fmt's printer to implement %#b and %O
to emit leading 0b and 0o prefixes on binary and octal.
(%#o is already taken and emits "0377"; %O emits "0o377".)

See golang.org/design/19308-number-literals for background.

For #19308.
For #12711.
Vet update is #29986.

Change-Id: I7c38a4484c48a03abe9f6d45c7d981c7c314f583
Reviewed-on: https://go-review.googlesource.com/c/160246


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>