Incorrect `EPropGuards` case in `TVars` instance for `Expr`

[qsctr]

The EPropGuards case in the TVars instance for Expr does not perform substitution inside the Type field of EPropGuards:

cryptol/src/Cryptol/TypeCheck/Subst.hs

Line 438 in 63f0ac6

EPropGuards guards ty -> EPropGuards !$ (\(props, e) -> (apSubst su `fmap'` props, apSubst su e)) `fmap'` guards .$ ty

This is causing declarations in prop guards whose type mentions a module type parameter to not get instantiated properly by moduleInstance, which causes the bug in GaloisInc/saw-script#1923.