Expose reserve on CxxString

[dtolnay]

This has come up as necessary in APIs that connect bytes-based code in Rust to std::string-based code in C++.

use bytes::Buf;
use cxx::let_cxx_string;

fn thing(buf: &mut dyn Buf) {
    let_cxx_string!(string = "");

    // handled by helper:
    string.as_mut().reserve(buf.remaining());
    while buf.remaining() > 0 {
        let chunk = buf.chunk();
        string.as_mut().push_bytes(chunk);
        buf.advance(chunk.len());
    }

    ffi::thing(string);
}

// ffi::thing:
// void thing(std::string& s) {
//   WriteMessage msg{std::move(s)};
//   ...
// }