Feature/strpool

[leissa]

This PR significantly speeds up the whole compiler. Impala test suite runs in 23s instead of 89s on my notebook (Debug build). And this time includes firing up clang and running the actual compiled program. Apparently, we spend much time on master packing and unpacking debug infos.

Def::meta is gone

Was only used at two spots:

1. Memorize field names for extract expressions a#x in the frontend. We are now simply using a map instead.
2. For keeping track of implicits. This is now a flags of a Pi instead which is much cleaner and simpler anyway.

Sym

All strings are tossed into a global hash set - the SymPool. This one lives in a new class Driver where a few global variables live. Well, there are not really global - that's the point of the Driver class. Also, it decouples some stuff from World and fights its god class status.

Sym simply wraps const std::string* (so pass it around as value) and does ==/!= comparisons in O(1). The empty string is internally handled as nullptr. Thus, you can create a Sym representing an empty string without having access to the SymPool. The empty string, nullptr, and "\0" are all identified as Sym().

Since creating a Sym has a slight runtime penalty (although much faster than the old solution), it's a good idea to create needed Symbols beforehand. You can see that in lower_for.h and a few other spots where I already did that but was too lazy to do it in all passes. It's only a slight penalty and hence probably hardly worth the effort. But if you feel like using this trick in your passes, the anonymous sruct and struct initialization - as I did in lower_for.h - are arguably the solution requiring the least amount of boilerplate.

Def::dbg

This changed from const Def* to struct Dbg { Loc loc; Sym sym }.

Instead of passing const Def* dbg all over the place, all classes now have setters:

def1->set(loc);
def2->set(loc, sym);
def3->set(sym);
def4->set(dbg);

Or directly set on creation:

auto lam = world.nom_lam(/*...*/)->set(loc); // lam has type Lam*

Misc

• Since I touched a lot of signatures I also changed those from const Def* to Ref.
• The introduction of the setters requires some macro magic which is a bit unfortunate but AFAIK there is no better way.
• see also optimization exhibits non-deterministic behavior #143
• Resolved some subtle Doxygen issues