Reorganize handling of task data

This abandons field-by-field compatibility with the TaskWarrior TDB2
format, which wasn't a sustainable strategy anyway.

Instead, tasks are represented as a TaskMap with custom key formats.  In
this commit, there are only a few allowed keys, with room to grow.

Replica returns convenience wrappers Task (read-only) and TaskMut
(read-write) with getters and setters to make modifying tasks easier.

docs/src/SUMMARY.md

@@ -6,5 +6,6 @@
 - [Data Model](./data-model.md)
   - [Replica Storage](./storage.md)
   - [Task Database](./taskdb.md)
+  - [Tasks](./tasks.md)
 - [Synchronization](./sync.md)
   - [Planned Functionality](./plans.md)