internal/number: rename Format to Pattern

Format is too overloaded and consists of more than
a pattern.

Change-Id: I92c08ca82930ef870e619c8fb7ebc6ef1b59bb69
Reviewed-on: https://go-review.googlesource.com/36278
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
5 files changed