Refactor backend interfaces

[b-studios]

Adresses #233

[b-studios]

There is still a few things to do:

• Merge Compiler and BackendCompiler into one trait
• Rename BackendRunner into Runner
• Drop generator.Backend
• Inline the pipelines of the backends into the respective compilers
• Move compilers to respective folders
• Factor out Backend file which serves as a main entry point for documentation.
• Move effekt.llvm to effekt.generator.llvm
• turn Stage into a GADT and abstract over pretty printing the intermediate IRs. this way we can cut down the interface to one, only.
• test with website

[b-studios]

It is starting to look better. One thing that I still don't like is the Either[String, Unit]... Maybe we should throw an exception instead?