Use jemalloc for allocation

[itamarst]

Benefits:

1. Consistent API across OSes
2. Might help with More robust fallback malloc() and friends on Linux #33, Make sure via test that we're not leaking memory due to using dynamic linker's malloc() #35
3. Might reduce memory fragmentation overhead

[itamarst]

Looks like it has the necessary symbol mangling: https://github.com/jemalloc/jemalloc/blob/master/INSTALL.md

[itamarst]

Spent some time talking to jemalloc developers, and reading code:

1. On macOS, libc code might use its internal malloc(), and then if user code uses public free() and it's jemalloc's free(), segfaults happen. As such, you can't safely use jemalloc as replacement allocator on macOS.
2. Mangled-namespace jemalloc for Rust wouldn't have this issue, because it'd be for Rust code only, and might provide a speed boost (no need to hit malloc() reentrancy path) and perhaps reduce fragmentation.
3. On Linux, there might be benefit to use jemalloc to reduce fragmentation and perhaps simplify the code so there's less dlsym() and pre-initialization edge cases.

[itamarst]

My first attempt at item 2 above (jemalloc for Rust) resulted in a deadlock or something, not sure why.