Add CResult to extra types crate

[fredszaq]

OK/KO type to be used as return type of extern(C) functions

Adding this should be a great opportunity to add some doc on a good pattern for error handling on a C API. (ie if the result was KO, then call a last_error function to get the actual error)

• type definition
• wrap function that takes a FnOnce returning a Resutl and returns OK or KO (and set last error in that case)
• last_error function to get the last error, that should be exposed in the FFI by the user of the crate
• document how to rename the struct in cbindgen. CResult makes a lot of sense in rust but not in C