Fix clippy lints 2025

[Nopey]

This PR cleans up the clippy lints, allowing some and addressing others.

I am developing on x86_64 Windows, so there's a chance I've overlooked lints in platform-specific places such as the ARM NEON code.

I have intentionally left the following 6 clippy lints unsolved, as they are addressed in sibling PR #121. (click to expand)

(warning and error are repeated 3 times each)

warning: this expression borrows a value the compiler would automatically borrow
    --> src\mem.rs:2055:5
     |
2055 |     (&mut vec[..up_to]).copy_from_slice(head);
     |     ^^^^^^^^^^^^^^^^^^^ help: change this to: `vec[..up_to]`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: calling `set_len()` immediately after reserving a buffer creates uninitialized values
    --> src\lib.rs:3319:9
     |
3319 | /         let mut vec: Vec<u8> = Vec::with_capacity(
3320 | |             (checked_add(
3321 | |                 valid_up_to,
3322 | |                 encoder.max_buffer_length_from_utf8_if_no_unmappables(string.len() - valid_up_to),
...    |
3325 | |             .next_power_of_two(),
3326 | |         );
     | |__________^
3327 |           unsafe {
3328 |               vec.set_len(valid_up_to);
     |               ^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = help: initialize the buffer or wrap the content in `MaybeUninit`
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec

warning: `encoding_rs` (lib) generated 6 warnings
error: could not compile `encoding_rs` (lib) due to 3 previous errors; 6 warnings emitted

Obsoletes open PR #78 ("Fix clippy lints") from 3 years ago.