Use wrapper functions VG_(malloc) and friends consistently across the

Use wrapper functions VG_(malloc) and friends consistently across the
board (instead of e.g. VG_(arena_malloc)(VG_AR_CORE,...). This change
also benefits static analysers. We can tell tools that VG_(malloc) allocates
and VG_(free) deallocates and that they are a pair. But we cannot do that for
arena_malloc/free.
Also provide a wrapper VG_(realloc_shrink).

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14517