all easy clippy fixes and cargo fmt

[robamu]

Those were some of the easier clippy fixes. I also ran cargo fmt.

There are a lot of result_unit-err lints like this one

warning: this returns a `Result<_, ()>`
   --> src/dec.rs:310:1
    |
310 | pub fn decode_in_place_with_sentinel(buff: &mut [u8], sentinel: u8) -> Result<usize, ()> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: use a custom `Error` type instead
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_unit_err
warning: this returns a `Result<_, ()>`

Those could either be disabled for the crate or a custom error type is introduced, which would be a breaking change..