`spec` is updated too.

It covers everything relevant to the lowering pass (unless I missed
something). The output uses a shorter formatting that doesn't match
what `pretty` produces -- a separate change for supporting S-expr
normalization in the tester will be necessary.

It's fully functional (according to the test suite), though not very
optimized, both in terms of output it produces and how it does so.

It's a run-of-the-mill test runner. Given the repetition, common parts
of the test runners should move into a separate module at one point.

The necessary runner support for disabling execution is still missing,
and the test would only result in an infinite loop.

The result stays the same, but how one gets there changes.

There are no "implicit parameter", only "implicit arguments". Attaching
the implicitness to the callee (i.e., target block) rather than the
caller (i.e., block exit) precludes some future optimizations, such as
merging basic blocks, in addition to just being wrong conceptually.

Only loops are iterated twice now, greatly reducing the time the pass
takes when there are no loops. Further optimizations are possible, such
as not following the back edge when there are no changes, but are not
implemented yet.

The documentation is also improved a bit.

A single-block `Loop` resulted in the forward propagation pass getting
stuck.

The test runner now supports the `--compileOnly` argument, which
disables validation and execution of the produced VM bytecode.

It didn't reflect reality.

This fixes the test failures.

Pinned locals are now only passed through to blocks/continuations that
perform an indirect memory access.

Procedure parameters must unconditionally be registered with the first
basic block.

Some of the pinned local tests trigger an illegal access error at run-
time, which is fine.

The new name more accurately describes which locals the set contains.

Some `.expected` files were not marked as storing S-expressions.

Co-authored-by: Saem Ghani <[email protected]>