Support runtime defined function wrapping of library module packed functions

[csullivan]

Targets which compile directly to SO during codegen utilize runtime.module.loadfile_so at runtime to dynamically open these compiled binaries. This flow currently does not have any point of extension in which a runtime can customize the behavior of the underlying LibraryModuleNode. This PR exposes DSOLibrary and extends LibraryModuleNode to utilize a runtime defined PackedFunctionWrapper in order to specialize the calling convention of dynamically loaded function symbols.

This allows a device runtime to then specify a custom DSOLibrary based loadfile implementation:

TVM_REGISTER_GLOBAL("runtime.module.loadfile_hexagon").set_body([](TVMArgs args, TVMRetValue* rv) {
  auto n = make_object<DSOLibrary>();
  n->Init(args[0]);
  *rv = CreateModuleFromLibrary(n, hexagon::WrapPackedFunc);
});

[tqchen]

Thanks @csullivan some followup comments and I think it is good to go. The main rationale behind is to minimize the API surface getting exposed. and some readability(internal documentation of wrapper semantics)

[tqchen]

dso_library.h

[tqchen]

depending on whether or not we need to expose this interface, we might want to put it back to the cc file to reduce the amount of internal interface. surface.

[csullivan]

As Library is an abstraction that is already exposed, I decided to reduce the interface to a helper that returns one instead.