* Make "none" DataType explicit

The None data type is created when converting an empty string to DataType.
Add functions to create it and recognize it. Convert it to the "void" LLVM
type in LLVM codegen.

* Rename "none" to "void"

* Map VoidType:Type -> Void:DataType in GetRuntimeDataType

* Map Void:DataType -> VoidType:Type in GetType