Log context changes

Removes dead code for context logging and adds:
- `lt.objs.context/enqueue`, a function that pushes an item to a queue
  unless the queue has reached its maximum predefined size.
  (Practically, a laxly defined ring buffer.)
- `lt.objs.context/context`, an object that stores `in` and `out`
  context buffers.
- `lt.objs.context/log`, a behavior that dispatches context changes to
  the relevant log buffer.

Caveat:
because context is managed by two unrelated functions, changes that are
always executed together are logged individually and not compounded.