Increase MaxBase from 36 to 62 and extend the conversion
alphabet with the upper-case letters 'A' to 'Z'. For int
conversions with bases <= 36, the letters 'A' to 'Z' have
the same values (10 to 35) as the corresponding lower-case
letters. For conversion bases > 36 up to 62, the upper-case
letters have the values 36 to 61.

Added MaxBase to api/except.txt: Clients should not make
assumptions about the value of MaxBase being constant.

The core of the change is in natconv.go. The remaining
changes are adjusted tests and documentation.

Fixes #21558.

Change-Id: I5f74da633caafca03993e13f32ac9546c572cc84
Reviewed-on: https://go-review.googlesource.com/65970Reviewed-by: Martin Möhrmann <moehrmann@google.com>