Minor code cleanups in vector.

Fixed a potential bug where calling replaceAt with a reference to
an existing element in the vector at the same index would cause
the element to be destroyed while being copied to itself.

Refactored the conditions in _grow and _shrink for clarity.
The computations are exactly the same but I think it reads better
this way.  In particular, the ssize_t variable 's' is gone: it didn't
need to be signed anyways because its value could never be negative.

Change-Id: If087841c15e6a87160eee874720c4a77eb0e99a6