Add an ambient Id substitution to Subst

After a struggle, I fixed Trac #5113 (again) on the 7.10 branch,
by adding an ambient substitution to Subst; see CoreSubst,
esp Note [IdSubstEnv].

This allowed me to do the impedence-matching in SimplEnv.substExpr
efficiently (fixing #10370) as well correctly (fixing the latest
problem with #5113).

This cost me more time than I like to say.  Sigh.