Minor API stabilization changes

[Lorak-mmk]

• Marked some enums as non_exhaustive, because we may want to add variants to them.
• Marked internal_testing module as doc(hidden), otherwise it would show up at docs.rs

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: 33c06a8

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 2764fc1d61ce83c4db5e0eff2db24dd2354b4454
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 2764fc1d61ce83c4db5e0eff2db24dd2354b4454
     Cloning 2764fc1d61ce83c4db5e0eff2db24dd2354b4454
    Building scylla v0.15.0 (current)
       Built [  35.256s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.051s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  34.292s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.048s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.117s] 127 checks: 126 pass, 1 fail, 0 warn, 0 skip

--- failure enum_marked_non_exhaustive: enum marked #[non_exhaustive] ---

Description:
A public enum has been marked #[non_exhaustive]. Pattern-matching on it outside of its crate must now include a wildcard pattern like `_`, or it will fail to compile.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#attr-adding-non-exhaustive
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/enum_marked_non_exhaustive.ron

Failed in:
  enum BatchStatement in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/statement/batch.rs:172
  enum TokenCalculationError in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/statement/prepared.rs:477
  enum PartitionKeyExtractionError in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/statement/prepared.rs:470

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  70.960s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.026s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.028s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.790s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.028s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.114s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  22.610s] scylla-cql
make: *** [Makefile:64: semver-rev] Error 1