try using the system allocator

[llogiq]

On r/rust, Alex Chrichton suggested we can use the system allocator.

This would presumably rid us of the 6MB upfront allocation of jemalloc, and make Rust look leaner in the benchmarks, so I'd like to benchmark this once I get to my PC.

[llogiq]

Note: This only works on nightly, so it doesn't work for us. However, configuring jemalloc might. I'm looking into that next.

[mati865]

With rust-lang/rust#51241 available since two releases (or three releases in few days) this should be considered again. If machine running the benchmarks has fairly recent glibc (at least 2.26) this could bring nice speedup.

[llogiq]

Apart from the binary_trees benchmark, which uses an arena anyway, none of the benchmarks allocate enough to make any difference, apart from reducing the base profile a bit.