Future proof number literals by eating longer suffixes.

### Summary

Tokenize number literal suffixes more eagerly to allow future
expansion. That is, tokenise a literal suffix as `[uif][0-9]*` rather
than the specific list we currently have.


[rendered](https://github.com/huonw/rfcs/blob/future-proof-number-literals/active/0000-future-proof-number-literals.md)