Derive `Bitfield` instances for `newtype`s generated for `typedef`s

[jorisdral]

If we generate bindings for:

struct myStruct {
  int x : 2;
};

And then compile the bindings, everything works.

If instead we generate bindings for:

typedef int myInt;
struct myStruct {
  myInt y : 2;
};

Then we get a GHC error that roughly says:

    • No instance for ‘HsBindgen.Runtime.Bitfield.Bitfield MyInt’
        arising from a use of ‘HsBindgen.Runtime.HasCField.peekCBitfield’

It should be simple to derive a Bitfield instance because MyInt is just a newtype around CInt.

Alternatively, we can disallow bit-field types like myInt. The C reference section on bit-fields seems pretty restrictive about what types are allowed as bit-fields, but implementations like gcc and clang seem to be more flexible.