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/

crates/default-alloc/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "minedmap-default-alloc"
+version = "0.1.0"
+description = "Helper crate for target-specific selection of global allocator default"
+edition.workspace = true
+license.workspace = true
+readme.workspace = true
+repository.workspace = true
+
+[dependencies]
+tikv-jemallocator = { version = "0.6.0", optional = true }
+
+[target.'cfg(target_env = "musl")'.dependencies]
+tikv-jemallocator = "*"
+
+[features]
+jemalloc = ["dep:tikv-jemallocator"]

crates/default-alloc/src/lib.rs

@@ -0,0 +1,3 @@
+#[cfg(any(target_env = "musl", feature = "jemalloc"))]
+#[global_allocator]
+static GLOBAL: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;