Add KernalArg instances for u64, i32, i64, and f32.
Add methods to manipulate Vector without allocations.

This adds two methods to Vectors.
`rewrite' updates the content of the vector without re-creating an openCL buffer.
`to_existing_vec' reads an openCL buffer and output the data on an user-provided vector. This
avoids useless allocations of a new rust vector.
Replaced copy by clone.
Merge pull request #4 from sebcrozet/master

Improve Vector reusability.