Add h3_to_string & h3_from_string

[vihu]

This PR attempts to add h3_to_string and h3_from_string functions to the library.

TODO:

• [ ] Figure out what's wrong with the tests (I could use some help here actually @rustyconover, since I haven't been able to run the tests locally either and they fail with the exact error as in the workflow build log here).
• [ ] It's likely that I messed something up with the Cstring FFI interface, presumably if we can get the tests running that can be fixed.