mpmc: feature-gate crossbeam implementation

Adds a feature-gate for the `mpmc::Queue` crossbeam implementation.

Author: 0xllx0

Cargo.toml

@@ -58,6 +58,9 @@ alloc = []
 
 nightly = []
 
+# Implement MPMC using `crossbeam::ArrayQueue`
+mpmc_crossbeam = ["dep:crossbeam-utils"]
+
 [dependencies]
 bytes = { version = "1", default-features = false, optional = true }
 portable-atomic = { version = "1.0", optional = true }
@@ -67,7 +70,7 @@ ufmt = { version = "0.2", optional = true }
 ufmt-write = { version = "0.1", optional = true }
 defmt = { version = "1.0.1", optional = true }
 zeroize = { version = "1.8", optional = true, default-features = false, features = ["derive"] }
-crossbeam-utils = "0.8"
+crossbeam-utils = { version = "0.8", optional = true }
 
 # for the pool module
 [target.'cfg(any(target_arch = "arm", target_pointer_width = "32", target_pointer_width = "64"))'.dependencies]