Implement mid level IR between YUL and LL

[xermicus]

Currently we translate YUL directly to LLVM. What I'm hoping to do better with an additional IR:

• Partially recover type information and use integer types narrower than i256 wherever possible
• Create the obvious place to implement other domain specific optimizations. For example eliminate unnecessary stack variables or recovering pointers and aliasing.

I'm eying to use MLIR (implies writing a YUL dialect). It is well designed and popular. A decent Rust wrapper crate already exists too.