…DIA#1232)

* thrust/mr: fix the case of reuising a block for a smaller alloc.

Previously, the pool happily returned a pointer to a larger oversized
block than requested, without storing the information that the block is
now smaller, which meant that on deallocation, it'd look for the
descriptor of the block in the wrong place. This is now fixed by moving
the descriptor to always be where deallocation can find it using the
user-provided size, and by storing the original size to restore the
descriptor to its rightful place when deallocating.

Also a drive-by fix for a bug where in certain cases the reallocated
cached oversized block wasn't removed from the cached list. Whoops.
Kinda surprised this hasn't exploded before.

* thrust/mr: add aliases to reused pointer traits in pool.h

…alloc