This change improves spirv-fuzz CMake code to be more compatible with other projects that might want to include spirv-fuzz as a sub-project.
* Add a CMake option for building spirv-fuzz.
* We now check if protobuf targets are already available.
* We no longer specify `-DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_USE_UNALIGNED=0`; a newer version of protobuf does not require this. Note that we probably should have specified this for protobuf targets as well, but this is no longer needed.
* Updated protobuf version in Kokoro scripts and README.md.
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.
