Latest clippy has spoken!

Fix the following errors reported by clippy:
```
error: manual arithmetic check found
  --> puzzlefs-lib/src/reader/puzzlefs.rs:45:27
   |
45 |           let addl_offset = if offset > file_offset {
   |  ___________________________^
46 | |             offset - file_offset
47 | |         } else {
48 | |             0
49 | |         };
   | |_________^ help: replace it with: `offset.saturating_sub(file_offset)`
      |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implicit_saturating_sub
   = note: `-D clippy::implicit-saturating-sub` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::implicit_saturating_sub)]`
```

and ignore the needless-lifetimes errors reported on the autogenerated
capnp code:

```
   |
15 |   impl <'a,> ::core::marker::Copy for Reader<'a,>  {}
   |         ^^                                   ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
   |
15 -   impl <'a,> ::core::marker::Copy for Reader<'a,>  {}
15 +   impl  ::core::marker::Copy for Reader<'_,>  {}
   |
```

Signed-off-by: Ariel Miculas-Trif <[email protected]>

puzzlefs-lib/src/compression/zstd_seekable_wrapper.rs

@@ -67,13 +67,13 @@ pub struct ZstdDecompressor<'a, R: Read + Seek> {
     uncompressed_length: u64,
 }
 
-impl<'a, R: Seek + Read> Decompressor for ZstdDecompressor<'a, R> {
+impl<R: Seek + Read> Decompressor for ZstdDecompressor<'_, R> {
     fn get_uncompressed_length(&mut self) -> io::Result<u64> {
         Ok(self.uncompressed_length)
     }
 }
 
-impl<'a, R: Seek + Read> Seek for ZstdDecompressor<'a, R> {
+impl<R: Seek + Read> Seek for ZstdDecompressor<'_, R> {
     fn seek(&mut self, offset: io::SeekFrom) -> io::Result<u64> {
         match offset {
             io::SeekFrom::Start(s) => {
@@ -97,7 +97,7 @@ impl<'a, R: Seek + Read> Seek for ZstdDecompressor<'a, R> {
     }
 }
 
-impl<'a, R: Seek + Read> Read for ZstdDecompressor<'a, R> {
+impl<R: Seek + Read> Read for ZstdDecompressor<'_, R> {
     fn read(&mut self, out: &mut [u8]) -> io::Result<usize> {
         // decompress() gets angry (ZSTD("Corrupted block detected")) if you pass it a buffer
         // longer than the uncompressable data, so let's be careful to truncate the buffer if it

puzzlefs-lib/src/format/types.rs

@@ -896,7 +896,7 @@ impl<'de> Deserialize<'de> for Digest {
     {
         struct DigestVisitor;
 
-        impl<'de> Visitor<'de> for DigestVisitor {
+        impl Visitor<'_> for DigestVisitor {
             type Value = Digest;
 
             fn expecting(&self, formatter: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {

puzzlefs-lib/src/lib.rs

@@ -12,6 +12,7 @@ pub mod fsverity_helpers;
 pub mod oci;
 pub mod reader;
 
+#[allow(clippy::needless_lifetimes)]
 pub mod metadata_capnp {
     include!(concat!(env!("OUT_DIR"), "/metadata_capnp.rs"));
 }

puzzlefs-lib/src/reader/puzzlefs.rs

@@ -42,11 +42,7 @@ pub(crate) fn file_read(
             continue;
         }
 
-        let addl_offset = if offset > file_offset {
-            offset - file_offset
-        } else {
-            0
-        };
+        let addl_offset = offset.saturating_sub(file_offset);
 
         // ok, need to read this chunk; how much?
         let left_in_buf = data.len() - buf_offset;

puzzlefs-lib/src/reader/walk.rs

@@ -48,7 +48,7 @@ impl<'a> WalkPuzzleFS<'a> {
     }
 }
 
-impl<'a> Iterator for WalkPuzzleFS<'a> {
+impl Iterator for WalkPuzzleFS<'_> {
     type Item = Result<DirEntry>;
 
     fn next(&mut self) -> Option<Self::Item> {