Update bitvec requirement from 0.19 to 0.20

Updates the requirements on [bitvec](https://github.com/myrrlyn/bitvec) to permit the latest version.
- [Release notes](https://github.com/myrrlyn/bitvec/releases)
- [Changelog](https://github.com/myrrlyn/bitvec/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/myrrlyn/bitvec/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Cargo.toml

@@ -76,7 +76,7 @@ features = [
 # Dependencies used when developing `turtle` directly, but not when using it as
 # a library.
 [dev-dependencies]
-bitvec = "0.19"
+bitvec = "0.20"
 
 # Since the debug performance of turtle isn't all that great, we recommend that
 # every user of turtle add the following to their Cargo.toml