Move ArrayHandleRuntimeVec metadata to a separate class

Originally, the metadata structure used by the ArrayHandleRuntimeVec storage was a nested class of its Storage. But the Storage is templated on the component type, and the metadata object is the same regardless. In addition to the typical minor issue of having the compiler create several identical classes, this caused problems when pulling arrays from equivalent but technically different C types (for example, long is the same as either int or long long).