Reorganize taskchampion crate for docs and tests

The public API of the taskchampion crate now contains the expected parts
and no more, and has some better documentation.

This moves the crate's external `tests/` into internal tests, as the
TaskDB is no longer exposed as part of the crate API.

taskchampion/src/taskstorage/inmemory.rs

@@ -1,5 +1,4 @@
-use crate::operation::Operation;
-use crate::taskstorage::{TaskMap, TaskStorage, TaskStorageTxn};
+use crate::taskstorage::{Operation, TaskMap, TaskStorage, TaskStorageTxn};
 use failure::{format_err, Fallible};
 use std::collections::hash_map::Entry;
 use std::collections::HashMap;
@@ -139,6 +138,8 @@ impl<'t> TaskStorageTxn for Txn<'t> {
     }
 }
 
+/// InMemoryStorage is a simple in-memory task storage implementation.  It is not useful for
+/// production data, but is useful for testing purposes.
 #[derive(PartialEq, Debug, Clone)]
 pub struct InMemoryStorage {
     data: Data,