Optimize the fast paths of calloc() and [m,d,sd]allocx().

This is a broader application of optimizations to malloc() and free() in
f4a0f32d34 (Fast-path improvement:
reduce # of branches and unnecessary operations.).

This resolves #321.

include/jemalloc/internal/private_symbols.txt

@@ -291,15 +291,11 @@ huge_ralloc
 huge_ralloc_no_move
 huge_salloc
 iaalloc
+ialloc
 iallocztm
 iarena_cleanup
-icalloc
-icalloct
 idalloc
-idalloct
 idalloctm
-imalloc
-imalloct
 in_valgrind
 index2size
 index2size_compute