Break cyclic reference between utils and platform.

Platform includes utils.h for the definition of Vector<>, so utils.h
can't include platform.h and has to use memcpy instead of OS::MemCopy.

We split out Vector<> into its own header to break this cycle
dependency.

BUG=none
R=mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/251753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20962 ce2b1a6d-e550-0410-aec6-3dcde31c8c00