internal/format: add API for symbol and digit info

Also exported SymbolType and related constants.

Change-Id: I0a934d315fe1a1e4ab803ddfa4fdcd1f8ea6c0be
Reviewed-on: https://go-review.googlesource.com/19198
Reviewed-by: Nigel Tao <nigeltao@golang.org>
6 files changed