Threading model needs a redesign

[JukkaL]

Currently the Alore threading model (basically the thread module) is poorly documented and too low-level. It needs a rethink, and things like the following need to be considered:

• Actor-based isolation model could be useful in addition to the low-level threaded model. Threading is still probably essential for getting good performance in cases when actors just don't work.
  • We might also need better support for immutable types.
• Make it legal to access differents parts of a single array in multiple threads concurrently, if they don't add/remove items.
• Add higher-level constructs such as futures or promises, message queues.

When work starts properly on this issue, this needs to be split into multiple issues.