[feature request] Make Cap and Join enums repr(u8)

[tower120]

That would allow use them as "array indices".

For example, skia-safe have them repr(u32) https://rust-skia.github.io/doc/skia_safe/paint/enum.Cap.html.

[waywardmonkeys]

I previously did this and more in #495. It was decided not to merge it in an office hours and I don’t remember why.