Add --target=pbc and use for build

[gerdr]

Adds a new option --target=pbc to the frontend. This is the first tiny step on the way to direct bytecode generation as envisioned by whiteknight.

The corresponding NQP changes can be found in the branch target-pbc of the main NQP repository, and the old Parrot-side infrastructure has been removed in whiteknight's branch eval_pmc.

Should not be merged into nom: please create a new branch instead.

Once someone pulls the trigger on parrot/parrot#934 , the code can be tested via perl Configure.pl --gen-parrot=eval_pmc --gen-nqp=target-pbc.

[gerdr]

@pmichaud I believe I've taken care of all of your complaints, so please take another look if you have a free minute.

The PackfileView PMC now comes with 2 additional methods first_sub_in_const_table() and single_sub_by_tag(). The former is the drop-in replacement for the old logic, is used right now but imo should go away long-term. The latter can only be used after tag generation has been added to the code generator. Right now, it's implemented in terms of subs_by_tag(), which should be fixed once it becomes more heavily used.

I don't think it's an immediate concern: I'd wait with switching to the new tag-based system until someone takes a shot at direct bytecode generation.

[gerdr]

Now obsolete - see #106