src: pass desired return type to allocators

Pass the desired return type directly to the allocation functions,
so that the resulting `static_cast` from `void*` becomes unneccessary
and the return type can be use as a reasonable default value for the
`size` parameter.

PR-URL: https://github.com/nodejs/node/pull/8482


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>