Shrink the PngError type

This brings `PngError` from 40 to 32 bytes on a 64-bit system, and
should only have a cost when a a `ReadFailed` or `WriteFailed` is
created

src/error.rs

@@ -15,9 +15,9 @@ pub enum PngError {
     InvalidData,
     InvalidDepthForType(BitDepth, ColorType),
     NotPNG,
-    ReadFailed(String, std::io::Error),
+    ReadFailed(Box<str>, std::io::Error),
     TruncatedData,
-    WriteFailed(String, std::io::Error),
+    WriteFailed(Box<str>, std::io::Error),
     Other(Box<str>),
 }
 

src/lib.rs

@@ -257,8 +257,9 @@ pub fn optimize(input: &InFile, output: &OutFile, opts: &Options) -> PngResult<(
             let output_path = path
                 .as_ref()
                 .map_or_else(|| input.path().unwrap(), PathBuf::as_path);
-            let out_file = File::create(output_path)
-                .map_err(|err| PngError::WriteFailed(output_path.display().to_string(), err))?;
+            let out_file = File::create(output_path).map_err(|err| {
+                PngError::WriteFailed(output_path.display().to_string().into_boxed_str(), err)
+            })?;
             if let Some(metadata_input) = &opt_metadata_preserved {
                 copy_permissions(metadata_input, &out_file)?;
             }
@@ -268,7 +269,9 @@ pub fn optimize(input: &InFile, output: &OutFile, opts: &Options) -> PngResult<(
                 .write_all(&optimized_output)
                 // flush BufWriter so IO errors don't get swallowed silently on close() by drop!
                 .and_then(|()| buffer.flush())
-                .map_err(|e| PngError::WriteFailed(output_path.display().to_string(), e))?;
+                .map_err(|e| {
+                    PngError::WriteFailed(output_path.display().to_string().into_boxed_str(), e)
+                })?;
             // force drop and thereby closing of file handle before modifying any timestamp
             std::mem::drop(buffer);
             if let Some(metadata_input) = &opt_metadata_preserved {

src/png/mod.rs

@@ -52,7 +52,8 @@ impl PngData {
     }
 
     pub fn read_file(filepath: &Path) -> Result<Vec<u8>, PngError> {
-        fs::read(filepath).map_err(|e| PngError::ReadFailed(filepath.display().to_string(), e))
+        fs::read(filepath)
+            .map_err(|e| PngError::ReadFailed(filepath.display().to_string().into_boxed_str(), e))
     }
 
     /// Create a new `PngData` struct by reading a slice