Fix rest of `clippy` errors and warnings and `#![deny(clippy::all)]`

[kkysen]

Right now we specify #![allow(clippy::all)], as we only want to lint against the most important safety lints:

• #![deny(clippy::undocumented_unsafe_blocks)]
• #![deny(clippy::missing_safety_doc)]

We'd like to run many more clippy lints of course, but only the safety-related ones were a priority.

A few lints we want to skip, though:

• #![allow(clippy::needless_range_loop)]
  We've paid careful attention to the performance of indexing loops, so places where an index is used instead of an iterator is done purposefully. When pre-sliced, the indexing version is often easier for LLVM to optimize.

• #![allow(clippy::too_many_arguments)]

Some other lints that we do want, but aren't enabled by default:

• #![deny(clippy::cast_lossless)]: See #1245.