Support for implementation of a Rust REPL

Makes a few changes that support the implementation of a REPL:

* Implementation of an intrinsic that returns the address of `__morestack`
* Implementation of wrapper code for LLVM ExecutionEngine API
* Fixing a change I made earlier to reset compiler state in `phase_1_[...]`
  instead of `compile_input` as the latter is not used in a REPL