Reland 20962 "Break cyclic reference between utils and platform."

Don't use OS::MemCopy in BitCast. It's crucial that the compiler can
optimize the memcpy away.

>  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

TBR=jkummerow@chromium.org
LOG=n
BUG=none

Review URL: https://codereview.chromium.org/252683004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20983 ce2b1a6d-e550-0410-aec6-3dcde31c8c00