vlib: convert frame_index into real pointers

The fast path almost always has to deal with the real
pointers. Deriving the frame pointer from a frame_index requires a
load of the 32bit frame_index from memory, another 64bit load of the
heap base pointer and some calculations.

Lets store the full pointer instead and do a single 64bit load only.

This helps avoiding problems when the heap is grown and frames are
allocated below vm->heap_aligned_base.

Type: refactor
Change-Id: Ifa6e6e984aafe1e2755bff80f0a4dfcddee3623c
Signed-off-by: Andreas Schultz <andreas.schultz@travelping.com>
Signed-off-by: Dave Barach <dave@barachs.net>