Support rust char binding, mapping to new cpp rust::Char

[kraktus]

Proof of concept to support the rust type char which is mapped to rust::Char.

This should be seen as the rust counterpart to #725, and address the other part of #592.

If this is approved, next steps/questions I can think of:

• Can we use the rust's char::from_u32 to validate the unicode value? (currently it's a copy of it)
• Can cpp's rust::Char be made a POD?
• What should be the scope of the API of cpp rust::Char? Currently intentionally minimal
• More tests (at least for each supported type in do_typecheck)
• Documentation