instead of enumerate the (long) list of platforms to exclude, use only
the short list of platforms to include.
should fixes __morestack symbol problem under openbsd
* Removes `RustJITMemoryManager` from public API.
This was really sort of an implementation detail to begin with.
* `__morestack` is linked to C++ wrapper code and this pointer
is used when resolving the symbol for `ExecutionEngine` code.
* `__morestack_addr` is also resolved for `ExecutionEngine` code.
This function is sometimes referenced in LLVM-generated code,
but was not able to be resolved on Mac OS systems.
* Added Windows support to `ExecutionEngine` API.
* Added a test for basic `ExecutionEngine` functionality.
Sébastien Marie