This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
* `debug.tls`
* `taskd.ca`
* `taskd.certificate`
* `taskd.ciphers`
* `taskd.credentials`
* `taskd.key`
* `taskd.server`
* `taskd.trust`
This uses CMake to build a simple Rust library (in `src/tc/rust`) that
just re-exports everything from the `taskchampion-lib` crate.
The C++ wrappers then wrap this into C++ objects with proper lifecycle
maintenance, in the `tc` namespace.
The C++ wrappers are incomplete, and missing methods are tagged with
"TODO". These will be added as needed.
