Performance improvements

[biqqles]

This branch contains several changes with the aim of improving initialisation performance considerably. Preliminary discussion is in #12.

Library changes:

• __init__ is now used for all initialisation, rather than __new__ and/or __call__.
  • As a side-effect, use post-initialisation logic no longer requires a special metaclass. DataClassInit has been removed
  • Post-initialisation logic is now accomplished by aliasing a user-defined __init__ to another name
• Default values are now looked up more efficiently

Together, these changes mean that dataclassy performs equally with dataclasses when slots=False, and far better when slots=True.

To do:

• [x] Decide whether to take this opportunity to support __post_init__
• [x] Write publishable benchmarks
• [x] Update documentation to reflect the above changes