make tests about one ore more functions

[piccolbo]

While assertions normally implement properties of certain functions that we want to test, this is fairly implicit in the qc api. A test takes an assertion, and such assertion invokes functions that need to operate properly for the assertion to succeed. Only in the argument cover can the user focus specifically on one function. This can make assertions less self documenting ("what was this assertion about"), but, more concretely, makes it harder to operate a choice of seed that makes test case selection independent between runs before and after a software fix (related to #26)

[piccolbo]

now test has argument "about", originally introduced to implement better seeding. But that doesn't answer the question: what are the tests for this function?

[piccolbo]

As a first step, return the about information with the test results. Then we need to gather that information to allow finding all the tests about a certain function.

[piccolbo]

At this point we have a test.set function that allows people to gather tests in group and see which functions are exercised by which tests. There may be better ways of printing out this information and there may be a need for tests that span across multiple files. The heuristic for the about arg needs to be battle tested.

[piccolbo]

need also to add tests for the new features and increase coverage

[piccolbo]

And documentation