Merging with thorin v2

[madmann91]

Here are the (incremental) goals to merge Thorin 2:

• Short term:

1. LLVM backend
2. Tuple params/app node/refactoring/merging types-Def, make debug info a Def
3. Serialization
4. Debugging infra
5. Modelling PrimOps with functions

• Long term:

6. Arbitrary PE filters
7. Optimization passes
8. Automatic differentiation
9. Garbage collection

[leissa]

Started working on 1) in the llvm_rewrite branch.

[leissa]

• 1 is 90% done.
• 4 is also 80% done but is subject to change as soon as we start working on 2)