Support c_char type

[dtolnay]

These should be implemented in terms of:

• char = https://doc.rust-lang.org/std/os/raw/type.c_char.html
• signed char = https://doc.rust-lang.org/std/os/raw/type.c_schar.html
• unsigned char = https://doc.rust-lang.org/std/os/raw/type.c_uchar.html

Useful for passing &[c_char] --> rust::Slice<char> --> const char*.

[dtolnay]

Maybe only char is needed? I guess signed char and unsigned char will always be the same as i8 and u8 on platforms that Rust supports.