Add a mechanism to skip a pass by name

[kripken]

For example, -O3 --skip-pass=vacuum will run -O3 normally but it will not run the
vacuum pass at all (which normally runs more than once).

The motivation for this is that J2Wasm wants to skip GlobalTypeOptimization in the
early cycles of optimization. We want to run -O3 a few times without GTO first, as
that preserves the shape of the vtables and itables (GTO removes and changes
fields). After that we'll apply an optimization for itables and vtables. And after that
optimization cycles can use GTO normally. Rather than add a special --skip-gto
flag, this PR adds a generic mechanism which could be useful for other things
later in principle.