Add jemalloc and jemalloc-auto features

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/

src/main.rs

@@ -2,6 +2,9 @@
 #![warn(missing_docs)]
 #![warn(clippy::missing_docs_in_private_items)]
 
+#[cfg(feature = "jemalloc-auto")]
+extern crate minedmap_default_alloc;
+
 mod core;
 mod io;
 mod util;