[vector] reshape, resize, rebracket, cast

[Brian Goetz]

I think the operations `reshape`, `resize`, `rebracket`, and `cast` 
could use some renaming, reshuffling, and rebracketing :)

Given the parameterization of vectors by (type, shape), a user might 
thin that "reshape" means "take a vector of E of shape S, and convert it 
to a vector of E of shape S'".  In other words, give it a new shape. 
But, that's not what it means -- it is a bit-level reinterpret, with 
truncation and fill.  The operation for "give a vector a new 
Vector.Shape" is actually called "resize".  This is not obvious, because 
of the multiple uses of "shape" in this API.

I'd recommend the following refactorings:

  - Rename current reshape to "reinterpret", since that is really what's 
going on -- the bits are being reinterpreted.
  - Rename current rebracket to "reinterpret" also, since it's the same 
operation -- just holding shape constant.
  - Rename "resize" to "reshape" (optional, resize is also OK)
  - Move all these operations from Species to Vector / XxxVector:

     class Vector<E> {
         Vector<E> reshape(Shape sh);               // or resize
         <F> Vector<F> reinterpret(Species<F> sp);  // was reshape
         <F> Vector<F> reinterpret(Class<F> cl);    // was rebracket
         <F> Vector<F> cast(Class<F> cl);           // was cast
     }

(I may have gotten the semantics of these wrong, but if so -- that only 
strengthens my point -- that its hard to keep them straight in the 
current stacking.)

Similar move for `cast(Mask)` and `cast(Shuffle)` -- move them to Mask 
and Shuffle.

Making them instance methods on Vector seems more natural -- it's an 
operation on the vector, and the species is just providing data to 
parameterize the operation.  (It also lays the groundwork for further 
refactoring Species.)