Extend from_enum macro to keep all enums at smallest repr

[itsjunetime]

The from_enum macro, before this PR, defined each macro to have variants of isize type. While this probably didn't really cause any issues in practice, this always made me a bit nervous. As far as I can tell, rust doesn't guarantee that enums with repr(rust) are automatically sized down to the smallest representation they could use (see the reference chapters on type layout and enums, maybe I missed something), so we might be passing all enums around as isizes right now (as opposed to their most optimal representation, which is a u8 in most cases).

So this PR, to fix that, allows specifying a repr for all enums created with from_enum and ensures that every constant passed in to be the value of a variant in that enum will actually fit within the given repr.

This pr also cleans up a few other small things, like using a fully-qualified path for $crate::error::Error and providing a way to convert from an enum into its c representation (e.g. c_int) losslessly (to ensure that we don't accidentally lose information when we add a new enum without checking what its largest value actually is).

[ginnyTheCat]

According to https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.discriminant.repr-rust the compiler is allowed to choose a smaller repr and does this in practice as well. Using a enum U2 { Zero, One, Two, Three } is quite common for example and the compiler not doing this optimization would "break" crates like smol_str by increasing their size and therefore removing the performance benefit they give.

Not sure about the repr part of this PR therefore. The rest looks good.