Replace Task & DB interface with C interface to TaskChampion

[djmitche]

TaskChampion now has a C interface via Rust FFI.

The next step is to integrate this into Taskwarrior:

• Link with TaskChampion
• Wrap the C API into C++ (smart pointers, method wrappers)
• Replace TF2 and TDB2 with this implementation

The result will be a Taskwarrior that uses a solid data backend with a secure sync protocol, and is interoperable with other tools also built on TaskChampion.

[djmitche]

I've gotten a start on this in #2771

[djmitche]

OK, I'm going to break this down a bit and land things one by one.

One major change is this: I'll take the code from TaskChampion and make it a part of TW. Then we don't have two "TaskThing"s causing confusion.