Allow C-like enum with a repr other than u8 to be Encode/Decode

[real-felix]

Hi, currently this code cannot compile:

#[derive(Clone, Copy, Debug, Decode, Encode, PartialEq)]
#[repr(u16)]
pub enum Status {
    Foo = 100,
    Bar = 200,
    Baz = 300,
}

Error message:

error: literal out of range for `u8`
  --> frame/foo/src/lib.rs
   |
   |     Baz = 300,
   |           ^^^
   |
   = note: the literal `300` does not fit into the type `u8` whose range is `0..=255`

It's because, as per the documentation,

The variable is encoded with one byte for the variant

but is there a reason to make this rule stands for the C-like enum?

[bkchr]

For now, I just propose that you write the Decode/Encode impl manually. Isn't that hard.