Add missing transformers/mtl instances.

This means instances for ExceptT, which deprecates ErrorT; for the
strict versions of WriterT and StateT; and for RWST (both versions).
Relax overly strict constraint.
Turn Rand into a type synonym.

This follows in the footsteps of other synonyms like Reader, Writer,
State and so on. This makes Rand automatically benefit from any instance
client code may provide for RandT.