Remote: support credentials via CFFI
Remote: add support for transfer and update_tips though CFFI
Remote: add documentation strings

Now that it has the features of the old implementation, let's add
documentation on how to use it.
Remote: get rid of the C code

This code has been obsoleted by the CFFI-using code. Some credentials
code remains in C due to the clone functionality making use of it.
Implement clone via CFFI as well

This lets us get rid of the last piece of C for anything related to
remotes and credentials.