Stubbed the functions in the Vector.Parallel module (analogue of Array.Parallel).
Started adapting the Array tests to Vector; this commit represents "in-progress" work and does not build.
Moved the vector type into Pervasives.fs since it should be considered a "core" type like array or list.
Moved the type aliases out of the AdditionalOperators module into the namespace.
Renamed the AdditionalOperators module to Operators to match the F# core library.
Created an ExtraTopLevelOperators module to correspond with the one in the F# core library; moved the tag/untag functions in there and added a 'vector' operator to simplify creation of a vector.