Add format to Integral types, similar to Double.format and SML's Int.fmt functions.

[crusso]

Most of this is already implemented in the prelude so its just a matter of exposing it properly, I think.

[ehoogerbeets]

No, please DO NOT add that! It will be English only that way. Implement a separate formatter that formats based on the locale and make posix/C or English/US the default locale for now. Now's your chance to make an international friendly language instead of falling into the same old trap that all previous programming languages have...

[rossberg]

@ehoogerbeets, this is supposed to generate Motoko number syntax, so language does not affect it. There is plenty of room to add i18n support to the platform, but that's different functionality.