TmOracle: Replace negative term equalities by refutable PmAltCons

The `PmExprEq` business was a huge hack and was at the same time vastly
too powerful and not powerful enough to encode negative term equalities,
i.e. facts of the form "forall y. x ≁ Just y".

This patch introduces the concept of 'refutable shapes': What matters
for the pattern match checker is being able to encode knowledge of the
form "x can no longer be the literal 5" or "x can no longer be Just y,
for any y". We encode this knowledge in the form of a `PmRefutEnv`,
storing a set of newly introduced `PmAltCon`s (literals and `ConLike`s)
for each variable denoting equations of the above form.

So, say we have `x ≁ Just ∈ refuts` in the term oracle context and
try to solve an equality like `x ~ Just 5`. The entry in the refutable
environment will immediately lead to a contradiction.

This machinery makes the whole `PmExprEq` business completely
unnecessary, getting rid of a lot of (mostly dead) code.

Note that the PmAltConLike case is currently unnecessary. This is bound
to change in a follow-up patch. If we began to use PmAltConLike, we'd
even profit from nicer error messages as is currently the case for
negative literal equalities.

See the Note [Refutable shapes] in TmOracle for a place to start.