Add lazy parallel map functions to Stream

I'm not sure if they should go into Stream or a new Par module, but considering there are only two functions and they're lazy I placed them in Stream here.

These functions are pretty basic mapping functions, but much of parallelism comes down to divide-map-reduce. The reduce part is trivial with out reduce based collections and divide is easy enough for the user to write. Only the mapping is complicated enough to warrant own functions.