Add subprojects; first pass at adding laws and tests.

This commit creates three sub-projects:

 - core: builds algebra.jar; contains type classes (and a few types)
 - std:  builds algebra-std.jar; contains sample instances
 - laws: builds algebra-laws.jar; contains laws and conformance tests

It also ports over the basic laws from Spire for:

 - signed
 - eq/order
 - groups
 - rings

Finally, it adds a very simple standard algebra instance for Int, and
runs the relevant laws tests on it.
(I ended up convincing myself this was right and just doing it. If this isn't good enough I'm willing to keep working on it.)