Don't use Arc in Settings unnecessarily

[dstu]

Per discussion in #871, it seems better for Settings to have an Rc internally instead of an Arc. This PR makes that change.

This passes make test check-msrv lint check-minver format-check && cargo run -p cargo-insta -- test.

Question for review: it would be simpler, and arguably better, if Settings just had fields instead of an inner Rc. This should leave the Settings interface as it is, since it uses simple interior mutation (fields would continue to be private with public accessors/mutators).

As-is, Settings has clone-on-write semantics, extra-cheap cloning (bumping a refcount copying fields), and extremely modest memory savings because newly constructed Settings objects share a set of immutable fields. But I'm not sure that the complexity imposed by having Settings wrap a pointer to an inner ActualSettings is justified. Is there a reason for using this pattern?

[john-h-kastner-aws]

FYI, this is a breaking change (per cargo semver-checks)

    Building insta v1.47.0 (current)
       Built [   0.636s] (current)
     Parsing insta v1.47.0 (current)
      Parsed [   0.009s] (current)
    Building insta v1.46.0 (baseline)
       Built [   4.836s] (baseline)
     Parsing insta v1.46.0 (baseline)
      Parsed [   0.008s] (baseline)
    Checking insta v1.46.0 -> v1.47.0 (minor change)

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/auto_trait_impl_removed.ron

Failed in:
     Checked [   0.006s] 196 checks: 194 pass, 2 fail, 0 warn, 56 skip
  type Redactions is no longer Send, in /tmp/insta-1.47/insta/src/settings.rs:26
  type Redactions is no longer Sync, in /tmp/insta-1.47/insta/src/settings.rs:26
  type SettingsBindDropGuard is no longer Sync, in /tmp/insta-1.47/insta/src/settings.rs:666
  type Settings is no longer Send, in /tmp/insta-1.47/insta/src/settings.rs:190
  type Settings is no longer Sync, in /tmp/insta-1.47/insta/src/settings.rs:190
  type Redaction is no longer Send, in /tmp/insta-1.47/insta/src/redaction.rs:53
  type Redaction is no longer Sync, in /tmp/insta-1.47/insta/src/redaction.rs:53

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/inherent_method_missing.ron

Failed in:
  Snapshot::matches, previously in file /home/jkastner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.46.0/src/snapshot.rs:516
  Snapshot::matches_fully, previously in file /home/jkastner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.46.0/src/snapshot.rs:525

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [   6.538s] insta

(discussion appears to be in #872)

[max-sixty]

is anyone affected by this? or it's just theoretical?

[john-h-kastner-aws]

There's a small break in our tests (cedar-policy/cedar-spec#917). We can work around it, but it's enough to block an automatic update.

[max-sixty]

ok. if you would like a reversion, happy to make it. sorry for the trouble

[max-sixty]

@john-h-kastner-aws I have reverted. Sorry again for the trouble

[max-sixty]

@dstu this makes the comparator slightly heavier. But the backward-compat is important, and seems like folks were relying on it. Hope that's OK.

[dstu]

Thanks for the heads-up. This doesn't appear to affect the insta-image crate, so things look good from that end.