This change adds a new test suite covering spinaBaseClassExtends()

(and a lot of common code shared with @spina).
In the process, some typing and runtime issues were found and fixed:


Mixins can now mix in static members.


Mixins are now applied before processing attributes, allowing for

  static members to be mixed in and processed.


An attribute was unnecessarily set twice when copying static members

  to the prototype.


extend() is now part of the SpinaClass interface.


Some functions now reference PartialSpinaClass rather than

SpinaClass, when working with a class that hasn't yet been fully

  processed.

Unit tests pass.