Gate dependency on crossbeam-channel behind the parallel feature

`crossbeam-channel` is only used if the `parallel` flag is enabled, so
it can be gated behind that feature flag to potentially reduce the size
of the build dependency tree for dependent projects.

While at it, I've fixed several warnings emmitted in tests/flags.rs.

Cargo.toml

@@ -29,10 +29,13 @@ indexmap = "1.9.1"
 libdeflater = "0.11.0"
 log = "0.4.17"
 stderrlog = { version = "0.5.3", optional = true, default-features = false }
-crossbeam-channel = "0.5.6"
 bitvec = "1.0.1"
 rustc-hash = "1.1.0"
 
+[dependencies.crossbeam-channel]
+optional = true
+version = "0.5.6"
+
 [dependencies.filetime]
 optional = true
 version = "0.2.17"
@@ -60,7 +63,7 @@ rustc_version = "0.4.0"
 [features]
 binary = ["clap", "wild", "stderrlog"]
 default = ["binary", "filetime", "parallel", "zopfli"]
-parallel = ["rayon", "indexmap/rayon"]
+parallel = ["rayon", "indexmap/rayon", "crossbeam-channel"]
 freestanding = ["libdeflater/freestanding"]
 
 [lib]

tests/flags.rs

@@ -4,6 +4,7 @@ use oxipng::{InFile, OutFile};
 #[cfg(feature = "filetime")]
 use std::cell::RefCell;
 use std::fs::remove_file;
+#[cfg(feature = "zopfli")]
 use std::num::NonZeroU8;
 #[cfg(feature = "filetime")]
 use std::ops::Deref;
@@ -95,9 +96,12 @@ fn test_it_converts(
 
 #[test]
 fn verbose_mode() {
+    #[cfg(feature = "parallel")]
     use crossbeam_channel::{unbounded, Sender};
     use log::{set_logger, set_max_level, Level, LevelFilter, Log, Metadata, Record};
     use std::cell::RefCell;
+    #[cfg(not(feature = "parallel"))]
+    use std::sync::mpsc::{channel as unbounded, Sender};
 
     // Rust runs tests in parallel by default.
     // We want to make sure that we verify only logs from our test.
@@ -169,7 +173,7 @@ fn verbose_mode() {
         thread_exec();
     });
 
-    let mut logs: Vec<_> = receiver.into_iter().collect();
+    let logs: Vec<_> = receiver.into_iter().collect();
     println!("logs={:?}", logs);
     assert_eq!(logs.len(), 9);
     let expected_logs = [