Add more Debug implementations

I generally like to implement Debug on these kind of types.

src/swash.rs

@@ -4,6 +4,7 @@
 use alloc::collections::BTreeMap as Map;
 #[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
+use core::fmt;
 #[cfg(feature = "std")]
 use std::collections::HashMap as Map;
 use swash::scale::{image::Content, ScaleContext};
@@ -96,6 +97,12 @@ pub struct SwashCache {
     pub outline_command_cache: Map<CacheKey, Option<Vec<swash::zeno::Command>>>,
 }
 
+impl fmt::Debug for SwashCache {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("SwashCache { .. }")
+    }
+}
+
 impl SwashCache {
     /// Create a new swash cache
     pub fn new() -> Self {