Add support for Rust char type

[bderrett]

Currently it doesn't seem to be possible to wrap a function accepting a char as an argument using PyO3. A char is a (particular kind of) unicode code point, so it could be converted to a single-character Python string. I think this would be a useful feature for users wanting to wrap such functions.

[kngwyu]

Good catch, thanks. For now, I made an initial implementation on #1282.

[bderrett]

Many thanks!