Start working towards NQPHLL cross-compilation.

This adds a HLL::Grammar with some pieces that don't yet work
commented out.
Compile uniprop (for /<:L>/ style things).

May need to do some massaging of the property names, but this basically
works.
Update ROADMAP.

Of note, all features of QAST::Regex are now handled.
Uncomment some Cursor methods that now compile.