C bindings

[Stranger6667]

We need C bindings to extend css-inline accessibility to other languages that can work via C.

TODO:

• Decide on the public API. Likely it is better to expose inline_to, so the caller is always responsible for managing their memory (otherwise we need to provide a separate function to deallocate the buffer we allocate during inlining with the inline function)
• Allow the end-user to configure the inlining process - not yet sure what is the best way to achieve this. E.g. it could be a struct that could be constructed from the C side.
• Generate a header file with proper extern declarations (cbindgen or similar as a starting point?)
• Document the bindings, specifically focusing on how the caller should manage memory when using this C API.
• Write tests in C that ensure our C API works and include them in CI

Notes:

• The bindings crate could live in bindings/c
• Validate the input from the C side (pointers should be not null)
• Maybe we can use some negative values to encode erroneous situations? (e.g. -1 for null pointers, -2 for invalid UTF-8, etc)
• Avoid allocating anything on our side
• Do not forget the null-terminators (\0) in the output.
• Consider using a custom Write impl

It can roughly look like this:

use libc::{c_char, c_int, size_t};
use std::ffi::CStr;
use std::ptr;

#[no_mangle]
pub extern "C" fn inline_to(input: *const c_char, output: *mut c_char, output_size: size_t) -> c_int {
    let cstr = unsafe { CStr::from_ptr(input) }.to_str().expect("TODO: handle properly");
    // ... Inline impl via `CSSInliner::inline_to`
}

// `Write` impl for a buffer coming from the C side?
use std::io::{self, Write};

struct CBuffer {
    buffer: *mut c_char,
    buffer_size: size_t,
    pos: usize,
}

impl CBuffer {
    fn new(buffer: *mut c_char, buffer_size: size_t) -> Self {
        Self {
            buffer,
            buffer_size,
            pos: 0,
        }
    }
}

impl Write for CBuffer {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        ...
    }

    fn flush(&mut self) -> io::Result<()> {
        ...
    }
}

cc @lobziik