Added support for structs to PHP.Core.Emit.Stelem()

Before this commit instantiating a class having (params SomeStruct[])
signature would result in an invalid argument error, because Emit didn't
handle the case when the type was *both* object and a value type.

This change seems to fix the problem.
Merge branch 'master' of https://github.com/DEVSENSE/Phalanger