Set up our own fuzzer

[rhuanjl]

We need fuzzing available - particularly for any significant changes to the JIT or type system.

I'm planning to look at getting Mozilla FunFuzz to work on ChakraCore: https://github.com/MozillaSecurity/funfuzz

Though I'm in interested in any other suggestions or offers to implement a fuzzer.

(Marked as Severity 1 as we can't/shouldn't make a release until various of the changes already in Master have been fuzzed)

[rhuanjl]

This issue: MozillaSecurity/funfuzz#223 (comment) appears to outline what would be needed to port funfuzz to run on a didfferent JS engine, I'll try and follow those steps this week.