Add bindings for adjacent fix-width numerical conversions and flesh out docs

[kentosugama]

Expose these: https://github.com/dfinity/motoko/pull/4139 Converting to adjacent fix-width types for:

• Int8

• Int16

• Int32

• Int64

• Nat8

• Nat16

• Nat32

• Nat64

And fleshing out the documentation for these modules to be up to standard with the Nat and Int modules.

[kentosugama]

Thank you for the review @luc-blaeser

[crusso]

Yes, nice work!

It's merged already but there's small typo in all the toText explanations. "Textual representation" -> "Textual representations". Also the reference to the original underscore syntax is because Motoko accepts underscores as thousand separators in literal (not commas). Probably not worth fixing.