Initial implementation of enum discrimnant sizing.

Allows an enum with a discriminant to use any of the primitive integer
types to store it.  By default the smallest usable type is chosen, but
this can be overridden with an attribute: `#[repr(int)]` etc., or
`#[repr(C)]` to match the target's C ABI for the equivalent C enum.

This commit breaks a few things, due to transmutes that now no longer
match in size, or u8 enums being passed to C that expects int, or
reflection; later commits on this branch fix them.