implement sallocx

[hdhoang]

hello again!

i have made an attempt at sallocx. I don't know how to build a C test program with custom _rjem_ prefix, so i copied existing v05 and call jemalloc-sys function in there. This requires pub fn alloc from the common directory.

about the test data, i don't understand how allocating 12 bytes, rounded up to 24:

  left: `24`,
 right: `16`', src/main.rs:12:9

naively, i thought 1->8, so 12->16 as the closest byte-multiple.

ps i think the "jemalloc-v05-unprefixed" test is uncommitted somehow.