Simplify codegen using template macro

[Virgiel]

Compared to a simple write! macro the code! macro has many advantages:

• Use a token stream instead of a string as input, we gain improved IDE support for free
• Native support for interpolating closure which gives many opportunities to reduce allocation by lazily writing directly into the output
• Native support for repetition and reuse iterators by cloning them which gives opportunities to reuse iterators and remove our hacky join trick

[LouisGariepy]

Aside from the two small comments I left and the clippy lints, this looks good to merge :+1:

[LouisGariepy]

Thanks for your work :+1: Merging!