Performance analysis

[flupe]

From my own personal use, achille has been pretty consistently fast. However, if I intend more people to use it, some time should be spent actually benchmarking the library and identifying causes for improvement.

Namely:

• The global cache is read fully, turned into a lazy bytestring and then propagated through tasks.
  Caches are typically small, but still maybe something like Conduit should be used to have more control over the space usage.

• Logging implementation is very naive so there is definitely room for improvement. I haven't looked into it yet as it's not critical, but it would be nice.