Fixes #175.

In order, the choice of repr is based on:

• #[repr(...)] attribute

  #[repr(u16)]
  enum E {  //:u16
      A,
      B,
  }

• otherwise, integer suffix

  enum E {  //:isize
      A = 0isize,
      B,
  }

• otherwise, range of discriminant values

  The first of u8, i8, u16, i16, u32, i32, u64, i64 that can fit all of the values.

  enum E {  //:i16
      A = -1,
      B = 2020,
  }

FYI @jgalenson