hydrant-ffi turns hydrant's default features off, so an embedded hydrant always ran on the embedding program's malloc, glibc for spindle on nixos, and never on the tuned jemalloc the binary uses. the new alloc-jemalloc feature, off by default, pulls in the same src/allocator.rs the binary and the benches use, linux tuning included. tikv-jemallocator's symbols are prefixed (_rjem_*), so with it on the staticlib carries jemalloc for hydrant's own rust allocations and defines no plain malloc, so the program's allocator is left alone. with it off the allocator module is empty and the library is the same as before.