TryFromPrimitive - accept more input types

[Crzyrndm]

I'm using this library to help generate a binary protocol so there's lots of repr(u8)/repr(u16) going around.

More often than not, the try_from input type is not a match for the repr type and particularly in the case of larger -> smaller types it is inconvenient to consistently need two try_from calls in a row that don't add anything

• TryFromPrimitive is going to match [0..3] whether the input is a u8 or a usize or anything in between. The conversion to u8 is just additional book-keeping (and one with an incompatible error type -.-)
• narrow->wide is better (generally) because of From but you still need to know the exact repr type so it's not perfect
• signed results from calculations and unsigned encodings are also common so it's not just widening/narrowing that creates papercuts

It makes sense to only handle the repr type in Into/From but limiting TryFrom just opens the door for as (which is the whole point of this crate to begin with)

Not sure what this would best look like (list of alternate try_from reprs? I have a couple of cases where types need different handling (e.g. u8 -> u16 shifts to the high byte) so opt-in would be better).