[vector] Remove generics from Vector.Shape

[John Rose]

On Jan 29, 2018, at 4:13 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> It's not clear to me why Shape needs to have any type variable which is bound to Vector. 

It seemed to me (when I first drafted it) that there would be
places where two vectors of different shapes would need to
interoperate, and where it would be useful to type-check
some aspect of the shape, such as its size or lane-type.

But it all comes down to details.  If shape parameters don't
add some value in static type checking, take them out.

Places to look for added value:  Rebracketing operations
like "change four ints to two doubles", sectioning operations
like "take upper two ints from four ints", reinterpreting
operations like "view four floats as four ints".

— John