Replace ~80% of code in `llvm_interpreter.rs` by just hand-writing a prelude

[bollu]

I'm using your project as a guinea pig (again). I want to see how successful this approach of storing the prelude as a .ll file is.

This will also help to add more stuff, because now it's just a raw .ll file that anyone can read & see.

I should be able to rip out a huge chunk of the source code that tries to generate this prelude, and focus on the actual compile step.

• [x] rip out prelude.ll
• [ ] load prelude.ll as an LLVM module
• [ ] JIT with external prelude
• [ ] what's the packaging story for prelude.ll? @polybuildr, I don't know how to package "assets" like these. Any ideas?