math/big: add %#b and %O integer formats

Matching fmt, %#b now prints an 0b prefix,
and %O prints octal with an 0o prefix.

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

For #19308.
For #12711.

Change-Id: I139c5a9a1dfae15415621601edfa13c6a5f19cfc
Reviewed-on: https://go-review.googlesource.com/c/160250


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