Fix all `clippy` warnings

[kkysen]

Fix all remaining clippy warnings. Then we can test cargo clippy in CI to keep it that way.

• vast majority of clippy warnings: Fix Clippy lints #460

• clippy::duplicate_underscore_argument
  Fix the clippy::duplicate_underscore_argument warning #497

• clippy::large_enum_variant
  Fix the clippy::large_enum_variant warnings that are simple by Boxing them #508
  Allow the last clippy::large_enum_variant warning #509

• clippy::type_complexity
  Fixed remaining clippy::type_complexity warnings by refactoring tuples into structs #467

• clippy::borrowed_box
  Fixed remaining clippy::borrowed_box warnings #476

• clippy::vec_box
  Locally allow clippy::vec_box warnings #499 (locally allowed)
  Fixing this would be a massive change. It cascades to basically remove every unneeded Box, which is generally good, but some of these types, Expr, Type, Item, are big, around 400 bytes, so the extra memcpys might not be worth it, but who knows. But also there are a ton of unnecessary allocations happening currently. The best possible solution would probably be to re-introduce a type like P, which was in libsyntax in rustc I believe, and which was arena-allocated. But syn uses Boxes, so we're kinda stuck as I don't think syn's types have an A: Allocator generic. The other alternative is just to #![allow(clippy::vec_box)], or at least do it locally on the few individual warnings. Maybe we should do that until more refactoring of the transpiler is done to de-duplicate a lot of it and add better APIs.

• clippy::if_same_then_else
  Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462
  For the remaining case, I'm not sure why this is the same in both branches. Should ask about before fixing, so it remains a warning for now.
  Fixed the clippy::if_same_then_else warning #498

• clippy::same_item_push
  Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462

• clippy::needless_bool
  Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462

[kkysen]

Fixing clippy::vec_box would be a massive change. It cascades to basically remove every unneeded Box, which is generally good, but some of these types, Expr, Type, Item, are big, around 400 bytes, so the extra memcpys might not be worth it, but who knows. But also there are a ton of unnecessary allocations happening currently. The best possible solution would probably be to re-introduce a type like P, which was in libsyntax in rustc I believe, and which was arena-allocated. But syn uses Boxes, so we're kinda stuck as I don't think syn's types have an A: Allocator generic. The other alternative is just to #![allow(clippy::vec_box)], or at least do it locally on the few individual warnings. Maybe we should do that until more refactoring of the transpiler is done to de-duplicate a lot of it and add better APIs.

[kkysen]

PRs for all clippy warnings have now been opened, with 3 left to merge. Once those all merge, we can close this issue and add clippy to #483.

[kkysen]

Fixed by:

• Fix Clippy lints #460
• Fix the clippy::duplicate_underscore_argument warning #497
• Fix the clippy::large_enum_variant warnings that are simple by Boxing them #508
• Allow the last clippy::large_enum_variant warning #509
• Fixed remaining clippy::type_complexity warnings by refactoring tuples into structs #467
• Fixed remaining clippy::borrowed_box warnings #476
• Locally allow clippy::vec_box warnings #499 (locally allowed)Locally allow clippy::vec_box warnings #499
• Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462
• Fixed the clippy::if_same_then_else warning #498
• Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462
• Fixed some of the remaining clippy warnings that needed slightly more complex refactors. #462