PR11334: stub out space for implementation of =~ operator.

PR11334: stub out space for implementation of =~ operator.

* staptree.h: define regex_query AST node and visitor behaviour.
* staptree.cxx: ditto.
* parse.cxx: parse nonassociative =~ at same precedence as comparison
             -- see parse_comparison_or_regex_query().
* elaborate.cxx: semantic check / tentatively dead code eliminate uses of =~.
* elaborate.h: ditto.
* translate.cxx: generate pointless stub code for =~.

* tapset/string.stp: stubs listing matched() functions et al for subgroup feature.

EMBARRASSINGLY OBVIOUS TODO: parse.cxx allows parsing !~, but doesn't
distinguish it from =~ in any fashion at the moment. Doesn't matter
since neither actually invokes a dfa in this commit!

NEXT STEP: gather the regexes we use in a global regcomp table.