SI-5903 extractor macros

Establishes a pattern that can be used to implement extractor macros
that give the programmer control over signatures of unapplications
at compile-time.

In particular, this power can be used to implement shapeshifting
pattern matchers for string interpolators without resorting to dirty
tricks. For example, quasiquote unapplications can be unhardcoded now.

Even though the pattern uses structural types, somehow no reflective calls
are being generated (as verified by -Xlog-reflective-calls and then
by manual examination of the produced code). That's a mystery to me, but
that's also good news, since that means that extractor macros aren't
going to induce performance penalties.

Almost. Unfortunately, I couldn't turn matchers into value classes
because one can't declare them local. Nevertheless, I'm leaving a canary
in place that will let us know once this restriction is lifted.

No modifications to core logic of typer or patmat are necessary,
as we're just piggybacking on https://github.com/scala/scala/pull/2835.

The only minor change I introduced is a guard against misbehaving
extractor macros that don't conform to the pattern (e.g. expand into
blocks or whatever else). Without the guard we'd crash with an NPE,
with the guard we get a sane compilation error.