fakevim: Increase/decrease octal and hexadecimal numbers

Hexadecimal (prefix '0x' or '0X') and octal (prefix '0') numbers are
unsigned. Decimal number can be signed (with leading '-').

Preserve width of hexadecimal and octal numbers (i.e. append leading
zeroes if result is shorter or remove leading zeroes if any and result
is longer).

Change-Id: I93769546ff40a586458986909fcee1e4e9143bc2
Reviewed-by: hjk <qthjk@ovi.com>