Update hashbrown requirement from 0.14.0 to 0.15.0 (#80)

Updates the requirements on [hashbrown](https://github.com/rust-lang/hashbrown) to permit the latest version.
- [Changelog](https://github.com/rust-lang/hashbrown/blob/master/CHANGELOG.md)
- [Commits](rust-lang/hashbrown@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: hashbrown
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Cargo.toml

@@ -20,7 +20,7 @@ ark-ff = { version = "0.4.0", default-features = false }
 ark-serialize = { version = "0.4.0", default-features = false, features = ["derive"] }
 ark-std = { version = "0.4.0", default-features = false }
 ark-poly = { version = "0.4.0", default-features = false }
-hashbrown = { version = "0.14.0" }
+hashbrown = { version = "0.15.0" }
 blake2 = { version = "0.10", default-features = false }
 rayon = { version = "1", optional = true }