Grammar preprocessing/compilation phase (after construction, before parsing)

[xparq]

Well, this is a huge big inconvenient question: the fully interpreted -- but less efficient -- mode, using the rules as-is, is mutually exclusive with running a precompiled version of the rules. Some of it could be merged, but the compiled version basically disqualifies self-modifying, or even just dynamically creating rules.

(At least eval (#28) could easily use the compiler, but even that feels so sad...)