Drop some uses of contextual when lexical will do.

This is good enough for the NQP subset of Perl 6, at least.
Only track lexicals in lowering optimization.
Optimize when regex calls back to the main lang.

This invokes the optimizer of the main language when there's a call
back into it. This in turn means we don't need to poison all lowering
of lexicals in the NQP optimizer in token/rule/regex. For now, we will
poision if we spot a call back into the main language; this can be
relaxed later.