Remove need for tuple_for_each!, tuple_len! and simplify gen_conditions! a lot

[poliorcetics]

By using named fields instead of anonymous one in the $mod_name::Streams structs, we can completely
remove the need for the very tuple_for_each! macros, which makes the code more maintainable and
the crate faster to compile (though that wasn't a limitation at this point).

I also took the opportunity to remove the tuple_len! macro because recursivity is heavy on the compiler. The generated code for this part doesn't slow down at all because it's all const-computed anyway (but instead of producing an imbalanced binary tree, we produce a more "normal" one which is much easier to deal with for rustc).

Finally, I greatly simplified the gen_conditions! macro by removing the recursion by using the Indexes enum again (that trick is awesome, I'm glad I finally found a crate to share it with 😄), which (again) removes a very unbalanced tree of 0 + 1 + 1 + 1 + ... in the generated code.

[poliorcetics]

For me it's a 20% improvement, probably because the generated code is much more optimiser-friendly because I don't think I did that much

critcmp main-branch remove-macro-branch

group              main-branch                            remove-macro-branch
-----              -----------                            -------------------
tuple::merge 10    1.21   1196.7±7.90ns        ? ?/sec    1.00    991.3±8.45ns        ? ?/sec

[matheus-consoli]

that's really awesome! thank you!