Clippy: allow or fix clippy::unnecessary_cast lint in src/

[DanielEScherzer]

In a lot of places, the lint is triggered when casting a constant from the libgit2 bindings that is created (in libgit2-sys) via git_enum!. That macro will use i32 when #[cfg(target_env = "msvc")], and u32 otherwise, meaning that a number of the casts that clippy flags as "unnecessary" are indeed necessary. In those cases, use #[allow] with a reason.

Similarly, clippy also lints against casting of c_int and c_uint to i32 and u32 - while those are usually the same types, the libc documentation makes it clear that they are not always the same. In those cases too, use #[allow] with a reason.

The only libc-provided type with a consistent rust type is that size_t is always the same as usize - fix the places that clippy identified where that conversion is unneeded. Other fixes include casts when the underlying type is explicitly set in the libgit2 bindings to be a specific rust type, or when converting mutable pointers to the same type they already are.

After these changes, running the command
cargo clippy -- -A clippy::all -W clippy::unnecessary_cast no longer reports any hits.

[DanielEScherzer]

This is the first of a few PRs to get clippy passing, and then I'll send a PR to add it to CI