Removes requirement for the compiler to be distributed with the
generator.
There is still a hard dependency on the precise OCaml version due to
syntactic changes between 4.02.1 and 4.02.2.

Also fix errors in type signatures in JsInterpreter.ml

This will permit us to run the test262 prelude in a separate context
from the testcase itself.
This was a bug in the original JSRef, see issue #9:
https://github.com/resource-reasoning/jscert_dev/issues/9