Use templates to reduce the redundant number-lexing functions

[Rangi42]

I've always been irritated by how we have so many of essentially the same function. Decimal numbers are still an odd one out because they don't have a prefix, but that was still tractable.

Ideally we'd be able to unify the rgbasm and rgblink functions, but their logic for how to read characters and handle errors is just too different. Maybe it could still be templated, but that would be too little gain for too much cost of extra abstraction layers.

This doesn't unify handling of fixed-point literals or graphics constants, because even though those are still "numeric literals" with "underscore digit separator handling", they're still too different to share logic. (Fixed-point literals have their whole fractional part, and graphics constants have two bitplanes and an 8-digit limit.)

Note that linkerscripts still have more lax parsing than assembly -- they don't enforce any rules about how '_' digit separators can be placed, and they don't warn for overflow (because rgblink has no -Wlarge-constant). I'll deal with that separately. (It's going to be tricky, because so far we only output warnings regarding object files, so all the warning-location-stack code assumes traversable FileStackNodes, which don't exist for included linkerscripts. Whee~)

[Rangi42]

Here's an example of how the templates get compiled: https://godbolt.org/z/34M3Gf5Tv

[ISSOtm]

Not a bad idea!