Fix clippy warnings

[traviscross]

Clippy is returning some warnings. Let's fix or explicitly ignore
them. In particular:

• In components/imageproc/src/lib.rs, we implement Hash explicitly
  but derive PartialEq. We need to maintain the property that two
  keys being equal implies the hashes of those two keys are equal.
  Our Hash implementations preserve this, so we'll explicitly ignore
  the warnings.

• In components/site/src/lib.rs, we were calling .into() on some
  values that are already of the correct type.

• In components/site/src/lib.rs, we were using .map(|x| *x) in
  iterator chains to remove a level of indirection; we can instead say
  .copied() (introduced in Rust v1.36) or .cloned(). Using
  .copied here is better from a type-checking point of view, but
  we'll use .cloned for now as Rust v1.36 was only recently
  released.

• In components/templates/src/filters.rs and
  components/utils/src/site.rs, we were taking HashMaps as
  function arguments but not generically accepting alternate Hasher
  implementations.

• In src/cmd/check.rs, we use env::current_dir() as a default
  value, but our use of unwrap_or meant that we would always
  retrieve the current directory even when not needed.

• In components/errors/src/lib.rs, we can use if let rather than
  match.

• In components/library/src/content/page.rs, we can collapse a
  nested conditional into else if let ....

• In components/library/src/sorting.rs, a function takes &&Page
  arguments. Clippy warns about this for efficiency reasons, but
  we're doing it here to match a particular sorting API, so we'll
  explicitly ignore the warning.

[Keats]

In which version was copied added? I'll have a look at the CI failures for sass...

[traviscross]

.copied went in (along with the updated clippy warning) in Rust v1.36.

rust-lang/rust#60333

[Keats]

Ah I would probably remove that change then, I usually target a couple of versions older than the latest stable (currently 1.34). I'll add that copied change in a couple of months

[traviscross]

That works; I've updated the branch to use .cloned() instead for now, which still resolves the warnings without depending on a recent version of rustc.

[traviscross]

Doing a clean build revealed some other clippy warnings that this branch now also fixes.

[Keats]

Thanks!