This commit removes some clippy warnings (to advance #45) by doing the following:
- replace usage of `mem::uninitialized()` with `MaybeUninit`
- replace usage of `nix::libc::{uint64_t, int32_t}` with `{u64, i32}`
- replace functions inside of `Option::ok_or` with `Option::ok_or_else`
- replace functions inside of `Result::unwrap_or` with `Result::unwrap_or_else`
- replace occurrences of pass-by-reference with pass-by-value when
appropriate
- replace unused variables in pattern-matching with wildcards
- replace `match` expressions that have only one case with `if let`
expressions
- replace UpperCamelCase names of consts with SCREAMING_SNAKE_CASE
- remove `clone()` on types that implement Copy
- remove redundant imports
- remove `fn main()` from doctests
- remove let binding for variables that are returned afterwards