puffin-client: rejigger error type (#1102)

This PR changes the error type to be boxed internally so that it uses
less size on the stack. This makes functions returning `Result<T,
Error>`, in particular, return something much smaller.

The specific thing that motivated this was Clippy lints firing when I
tried to refactor code in this crate.

I chose to achieve boxing by splitting the enum out into a separate
type, and then wiring up the necessary `From` impl to make error
conversions easy, and then making `Error` itself opaque. We could expose
the `Box`, but there isn't a ton of benefit in doing so because one
cannot pattern match through a `Box`.

This required using more explicit error conversions in several places.
And as a result, I was able to remove all `#[from]` attributes on
non-transparent error variants.

crates/puffin-distribution/src/source/mod.rs

@@ -754,17 +754,17 @@ impl<'a, T: BuildContext> SourceDistCachedBuilder<'a, T> {
 
         // Create a temporary directory.
         let temp_dir = tempfile::tempdir_in(self.build_context.cache().root())
-            .map_err(puffin_client::Error::CacheWrite)?;
+            .map_err(puffin_client::ErrorKind::CacheWrite)?;
 
         // Download the source distribution to a temporary file.
         let mut writer = tokio::io::BufWriter::new(
             fs_err::tokio::File::create(temp_dir.path().join(source_dist_filename))
                 .await
-                .map_err(puffin_client::Error::CacheWrite)?,
+                .map_err(puffin_client::ErrorKind::CacheWrite)?,
         );
         tokio::io::copy(&mut reader, &mut writer)
             .await
-            .map_err(puffin_client::Error::CacheWrite)?;
+            .map_err(puffin_client::ErrorKind::CacheWrite)?;
 
         Ok(temp_dir)
     }