Add a (very) basic performance microbenchmark.

(in the spirit of satisfying my curiosity)

This compares libffi to a direct external call that registers all GC roots,
bypasses this registration and uses a very-low-level noalloc.  This test
requires ctypes to be installed via ocamlfind currently.

Results on my Mac:
```
direct_add_slow        : 1.5174
direct_add_fast        : 0.7539
direct_add_fast_noalloc: 0.5897
indirect_add           : 14.2459
```

Which is roughly an order of magnitude worse with libffi for the worst-case benchmark. Sounds about right, and still pretty good.  I'll peer at it under perf later on Linux to see if there are any low-hanging speedups.