These functions all plug into RandomnessSource and produce random values
of various types. They are to be used either inside other generator
definitions or inside the GEN(...) macro when used in tests.
This will be a foundational part of bootstrapping generators: this is
the way they'll get prerecorded values from / record random values into
RandomRuns. (Generators don't get in contact with RandomRuns
themselves, they just interact with the RandomnessSource.)
To be used later by generators and during shrinking.
The generators used in randomized tests will record the drawn random
bits into a RandomRun. This is a layer of indirection that will help us
automatically shrink any generated value without any user input. (Having
to provide manually defined shrinkers is a downside to many randomized /
property-based testing libraries like QuickCheck for Haskell.)
