Adapt Buttercup to new libfuzzer-compatible languages

[michaelbrownuc]

Buttercup currently only supports C/C++ and Java, a relic of the competition. Buttercup's vuln-discovery component is OSS-Fuzz, and can support multiple languages. For compiled languages, if they are libfuzzer compatible they can be incorporated more easily than other languages.

We need to:

• [ ] find exemplar targets in languages like Go
• [ ] adapt Buttercup to handle them
• [ ] fix issues resulting from testing them