[WIP] CMakify the fuzzing directory

[derwolfe]

This is intended to make adding, updating, and maintaining fuzzers a bit easier.

• It makes it so fuzzers are built and run via cmake and make and exercised by CI
• It adds a unit test for the current fuzzer, while also making it possible to automatically add ctest tests for any new fuzzers that are added.
• It adds documentation detailing how fuzzers can be tested via oss-fuzz tooling and how to add new testcases.

I might move build.sh back out to oss-fuzz as it seems like it is kind of here without any real context or need.

[coveralls]

Coverage remained the same at 85.924% when pulling 968bdd0 on derwolfe:cmakify-fuzzers into 4badbe9 on json-c:master.