Allow enum discriminants to not be uint, and use smallest possible size by default.

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.

Also adds a lint pass for using non-FFI safe enums in extern declarations, checks that specified discriminants can be stored in the specified type if any, and fixes assorted code that was assuming int.