Use evaldUnfolding for proxyHashId

Edward and I noticed this - like realWorld#, we want things applied to
proxy# which are INLINE more likely to be inlined - we do this by
marking proxy# as having an evaldUnfolding, which in turn makes it look
like the primitive is evaluated and makes the simplifier more willing to
do so.

See Note [evaldUnfoldings] in MkId.

Signed-off-by: Austin Seipp <austin@well-typed.com>