We should be able to parse any .fildesh script without crashing, deadlocking, or entering an infinite loop.

• No more bailouts in parse phase! Part of No more bailouts! #35.
• Create //src:fildesh_builtin_lib that's everything but the real main(). We'll have to split main() out to a main.c.
• Parse scripts into an AST-like structure before interpreting their contents #86
• Give fildesh a main function that has FildeshX** and FildeshO** arrays alongside argv. We can use it to pass in data without actually doing file i/o.
• Add -parse flag that just parses the script. Includes are not exercised.
• Fuzz test fildesh -parse input.fildesh by calling the main function directly, using a FildeshX to hold the input.fildesh fuzz bytes.
• Make the fuzzer run cleanly. The parse phase doesn't actually do much, so hopefully this isn't much trouble.