AutogenConfig specified on multiple levels

[AlexeyRaga]

Currently we can supply AutoGenConfig on two levels:

1. Using Properties attribute on a class/module
2. Using Property attribute on a specific test

Current behaviour is that the config that is specified by Property attribute "wins".

I suggest that we add the ability to "merge" configs so that it'd be possible to supply a bunch of generators with Properties attribute, but the AutoGenConfig that is supplied for an individual test would be "appended" to the upper-level config.

This would probably mean that we make AutoGenConfig a semigroup, which has its own advantages, too.

[dharmaturtle]

That's a good idea. I'd want Hedgehog.Experimental to support AutoGenConfig.merge first though, since the generator collection is private. Once that's in place, I can add this feature rather easily.