[OCaml] Rework Llvm_executionengine using ctypes.

Since JIT->MCJIT migration, most of the ExecutionEngine interface
became deprecated and/or broken. This especially affected the OCaml
bindings, as runFunction is no longer available, and unlike in C,
it is not possible to coerce a pointer to a function and call it
in OCaml.

In practice, LLVM 3.5 shipped completely unusable
Llvm_executionengine.

The GenericValue interface and runFunction were essentially
a poor man's FFI. As such, this interface was removed and instead
a dependency on ctypes >=0.3 added, which handled platform-specific
aspects of accessing data and calling functions.

The new interface does not expose JIT (which is a shim around MCJIT),
as well as the interpreter (which can't handle a lot of valid IR).

Llvm_executionengine.add_global_mapping is currently unusable
due to PR20656.