go/scanner: accept new Go2 number literals

go/scanner: accept new Go2 number literals

This CL introduces go/scanner support for the new binary and octal integer
literals, hexadecimal floats, and digit separators for all number literals.
The new code is closely mirroring the respective code for number literals in
cmd/compile/internal/syntax/scanner.go.

R=Go1.13

Updates #12711.
Updates #19308.
Updates #28493.
Updates #29008.

Change-Id: I5315c6aaa7cfc41a618296be20e3acd5114d6b3c
Reviewed-on: https://go-review.googlesource.com/c/159997
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>