Fix most of `clippy::pedantic`

Part of #7.

This MR has fixed many of the clippy::pedantic lints, in alphabetical order.

Due to the huge refactor involved in the latest commit (49a917ba), the clippy::pedantic lints will be fixed in multiple MRs.

There are no interesting commits, just a lot of code being moved around.

rr_frontend/.cargo/config.toml

@@ -32,7 +32,6 @@ rustflags = [
     "-Aclippy::string_lit_as_bytes",
 
     # clippy::pedantic
-    "-Aclippy::map_unwrap_or",
     "-Aclippy::match_same_arms",
     "-Aclippy::must_use_candidate",
     "-Aclippy::needless_continue",