GenDefault class to derive generators

[ejconlon]

Implements the design in #49 . Defines GenDefault class to derive generators with support for user-defined tags to select implementations. Also includes various DerivingVia helpers.

[edsko]

Apologies for the slow reaction, I've been so busy. But this is really nice work! I like it. Thank you for your attention to detail. I think the only thing we should consider how we could make Test.Falsify.Generator.Default and Test.Falsify.Generator.Tag not be siblings, but instead clarify in the module hierarchy that one is an instance of the other. Perhaps Test.Falsify.Generator.Default and Test.Falsify.Generator.Default.Falsify? That double Falsify is a tad annoying but it does make sense. Maybe even Test.Falsify.GenDefault and Test.Falsify.GenDefault.Falsify`?

Alternatively, maybe, Test.Falsify.Generator.ClassBased and Test.Falsify.Generator.ClassBased.Default? That's perhaps a little cleaner, what do you think?

But once we resolve that, this is ready to merge. Sorry again for the delay!

[ejconlon]

@edsko You presented a few good options. What do you think about Test.Falsify.GenDefault and Test.Falsify.GenDefault.Std? We can change the name of the default tag to Std as well, to indicate that this is the standard set of generators. These are short module names that match their contents, so I think they will be easy for people to remember.

[edsko]

Yes, I like it :) Thank you!

[ejconlon]

@edsko Oops, forgot to add to exposed modules! (I use hpack for everything else...) #64