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

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?

The issue names the Encode and Decode derives and links their enum documentation, but it provides no implementation file or test path. Start by locating the derive handling for C-like enums and determine how repr types are currently interpreted. Done should define and test support for repr(u16) and other non-u8 representations without leaving the encoding behavior ambiguous.