This PR extends the RandomGenerator interface and fixes the
PseudoRandomGenerator class. It:
- Fixes a problem that made the RandomUint32 of PseudoRandomGenerator
segfault.
- Adds the RandomUint64 function to RandomGenerator and
PseudoRandomGenerator.
Fixes#3805.
Adds a library for spirv-fuzz, consisting of a Fuzzer class that will
transform a module with respect to (a) facts about the module provided
via a FactManager class, and (b) a source of random numbers and
parameters to control the transformation process provided via a
FuzzerContext class. Transformations will be applied via classes that
implement a FuzzerPass interface, and both facts and transformations
will be represented via protobuf messages. Currently there are no
concrete facts, transformations nor fuzzer passes; these will follow.
