Update chrono, and include fewer deps
Somewhere in the process of moving TaskChampion back to its own repo,
Taskwarrior's `Cargo.toml` ended up containing all of its dependencies,
unnecessarily.
This included an old version of `chrono` (addressed in
https://github.com/GothenburgBitFactory/taskchampion/pull/399). Removing
the old version requirement from `Cargo.toml` results in using a newer
version, addressing a (benign) security vulnerability.
* move taskchampion-lib to src/tc/lib, remove the rest
* update references to taskchampion
* Use a top-level Cargo.toml so everything is consistent
* apply comments from ryneeverett