[runtime] Use mono_assembly_get_object_handle instead of mono_assembly_get_object_che...

[runtime] Use mono_assembly_get_object_handle instead of mono_assembly_get_object_checked everywhere in the runtime. (#4061)

* [runtime] Move domain icall declarations to appdomain-icalls.h

* [coop] Use handles for System.AppDomain.GetAssemblies icall

* [coop handles] Add HANDLE_FUNCTION_RETURN_REF macro

* [runtime] Handles for AppDomain.LoadAssemblyRaw and Assembly.LoadFrom

* [reflection] Coop handles for RuntimeTypeHandle.GetAssembly

* [runtime] Change mono_try_assembly_resolve to return MonoAssembly

  instead of managed MonoReflectionAssembly.  Update call sites.

  Also implement it in terms of mono_try_assembly_resolve_handle (which
  also returns MonoAssembly*, not a managed object)

* [runtime] Use handles for System.AppDomain.LoadAssembly

* [reflection] Use mono_assembly_get_object_handle instead of mono_assembly_get_object_checked

  Convert all its callers to use handles and get rid of mono_assembly_get_object_checked