B::Deparse: stub implementation of deparsing lexical subs

This doesn't work properly, but (a) it's better than nothing, and (b) it
suppresses some unsightly "unexpected OP_INTROCV" warnings from the test
suite, fixing RT #116821.
deparse.t: delete now-unneeded __WARN__ suppression

B::Deparse no longer emits the warnings in question.
B::Deparse: document that `state sub` is unimplemented