Edge cases like base 2 and 36 conversions are now covered.
Many tests are mirrored from the itoa tests.

Added more test cases for syntax errors.

Change-Id: Iad8b2fb4854f898c2bfa18cdeb0cb4a758fcfc2e
Reviewed-on: https://go-review.googlesource.com/2463Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>