Have a variant of `JSRef` which lacks a type parameter?

[mgsloan]

The idea is to have something like this:

#ifdef ghcjs_HOST_OS
data JSRef' = JSRef' ByteArray#
#else
data JSRef' = JSRef' Addr#
#endif

newtype JSRef a = JSRef JSRef'

Why is this better? Because you can't use Coercible a (JSRef b) when b is ambiguous. It occurs to me that this constraint ought to work due to the type parameter of JSRef being phantom, but perhaps this is an issue with the implementation of Coercible (on GHC 7.8, anyway).

Anyway, even if this is a GHC bug, it seems cleaner to have this separation.