Handle platforms where c_int is not i32

[robin-nitrokey]

This PR adds a CI job that tests compatibility with an avr target (16-bit) and fixes code that assumes that c_int is always i32.

Replaces:

• Fix compile errors if c_int is not i32 on some platforms #68

One remaining problem is that some functions cast u32 return values to usize. In most cases that should be fine because the return value is limited by the length of the input buffer and thus fits into a usize. One notable exemption is seek. But let’s review and fix that separately.

[sosthene-nitrokey]

LGTM but the test fails because it's missing the std source I think

[robin-nitrokey]

Yeah, since rebasing onto main the CI fails, but the same commands worked before. Not sure why it fails now:

error: "/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/Cargo.lock" does not exist, unable to build with the standard library, try:
        rustup component add rust-src --toolchain nightly-x86_64-unknown-linux-gnu

But we already installed rust-scr for that toolchain:

rustup toolchain install --profile minimal --component=rust-src nightly
info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
info: latest update on 2024-08-08, rust version 1.82.0-nightly (8b3870784 2024-08-07)
info: downloading component 'cargo'
info: downloading component 'rust-src'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: installing component 'cargo'
info: installing component 'rust-src'
info: installing component 'rust-std'
info: installing component 'rustc'

  nightly-x86_64-unknown-linux-gnu installed - rustc 1.82.0-nightly (8b3870784 2024-08-07)

[robin-nitrokey]

And another rebase fixed it. 🤔