We were failing to emit wild-card hole constraints altogether
in the case of pattern bindings.  Reason: it was done in
tcExtendTyVarEnvFromSig, which isn't called for pattern bindings.

This patch make it work right for both pattern and function
bindings.  Mainly, there is a call to emitWildCardHolds in
tcRhs for both PatBind and FunBind.

I also killed off TcExpr.typeSigCtxt.