Fix clippy warnings in decoder and reader modules

- Replace manual arithmetic check with saturating_sub
- Remove unused lifetime parameter in Reader<Mmap> impl
- Remove unused import in test module

Author: oschwald

src/maxminddb/decoder.rs

@@ -55,7 +55,7 @@ impl<'de> Decoder<'de> {
             return size;
         }
 
-        let bytes_to_read = if size > 28 { size - 28 } else { 0 };
+        let bytes_to_read = size.saturating_sub(28);
 
         let new_offset = self.current_ptr + bytes_to_read;
         let size_bytes = &self.buf[self.current_ptr..new_offset];

src/maxminddb/lib.rs

@@ -209,7 +209,7 @@ pub struct Reader<S: AsRef<[u8]>> {
 }
 
 #[cfg(feature = "mmap")]
-impl<'de> Reader<Mmap> {
+impl Reader<Mmap> {
     /// Open a MaxMind DB database file by memory mapping it.
     ///
     /// # Example

src/maxminddb/reader_test.rs

@@ -179,7 +179,6 @@ fn test_reader_readfile() {
 #[test]
 #[cfg(feature = "mmap")]
 fn test_reader_mmap() {
-    use crate::Mmap;
     let _ = env_logger::try_init();
 
     let sizes = [24usize, 28, 32];