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.
Yeah I am totally up for improving this, but I'd rather get things merged first -- it will make it easier for many of us to hack on the specific instances.

We may decide that the `std` package ends up being more than just "demo/test instances" in which case we should definitely incorporate as much intelligence as possible.