Introduce the new features jemalloc (set jemalloc global allocator
unconditionally) and jemalloc-auto (set jemalloc global allocator on
musl-based targets to fix multithreaded performance, see [1]).
Because cargo does not support target-specific features or feature
defaults, the default is handled using a helper crate
minedmap-default-alloc.
[1] https://nickb.dev/blog/default-musl-allocator-considered-harmful-to-performance/
