Vars

[leissa]

This is sth I wanted to do for years, but I never really figured out how to do that exactly:
I want to get rid of thorin::Scope and, instead, work with free Vars. This is a major milestone with the new subsystem working but the old stuff is still in place - although I've already ported some of it. So, expect more PRs to come.

• Def::local_muts()/Def::local_vars() are Vars/mutables reachable by following immutable Def::extended_ops().
• Def::local_muts()/Def::local_vars() are cached and hash-consed.
• Def::free_vars() compute a global solution, i.e., by transitively following mutables as well.
• Def::free_vars() are computed on demand and cached.
  They will be transitively invalidated by following fv_consumers(), if a mutable is mutated.
• Def::is_closed()/Def::is_open() to check whether a Def contains free vars.

Change log

• removed ScopeRewriter in favor of new free_var-based VarRewriter
• simplifed rewrite interface
• PooledSet and Pool to hash-cons and arena-allocate sets
• sanity check whether externals are closed
• fixed issue in cps2ds.cpp but one issue remains (cps2ds sometimes introduces ownerless Vars #271)
• minor bug fixes/additions here & there

Future directions

• lazy rewrite/substitution (only substitute one mutable and a time and continue substituting as needed to prevent quadratic runtime when resolving curried beta-reduction etc)
• completely remove thorin::Scope
• new textual outputs
  • a simple output that dumps everything up to mutables
  • a proper "decompilation" of the thorin Graph into a thorin AST

[leissa]

@fodinabor, @NeuralCoder3 bump