Partitioner visibility

[wprzytula]

Clone of #1222.

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: 6d521b2

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev f6d37a628c2cef393fe7e13f8b2fc4220c0154fd
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev f6d37a628c2cef393fe7e13f8b2fc4220c0154fd
     Cloning f6d37a628c2cef393fe7e13f8b2fc4220c0154fd
    Building scylla v0.15.0 (current)
       Built [  24.655s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.048s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  21.985s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.046s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.124s] 127 checks: 124 pass, 3 fail, 0 warn, 0 skip

--- failure function_requires_different_generic_type_params: function now requires a different number of generic type parameters ---

Description:
A function now requires a different number of generic type parameters than it used to. Uses of this function that supplied the previous number of generic types will be broken.
        ref: https://doc.rust-lang.org/reference/items/generics.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/function_requires_different_generic_type_params.ron

Failed in:
  function calculate_token_for_partition_key (1 -> 0 generic types) in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/routing/partitioner.rs:351

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself 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/v0.39.0/src/lints/struct_missing.ron

Failed in:
  struct scylla::routing::partitioner::Murmur3Partitioner, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:100
  struct scylla::routing::partitioner::Murmur3PartitionerHasher, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:115
  struct scylla::routing::partitioner::CDCPartitionerHasher, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:283
  struct scylla::routing::partitioner::CDCPartitioner, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:281

--- failure trait_missing: pub trait removed or renamed ---

Description:
A publicly-visible trait cannot be imported by its prior path. A `pub use` may have been removed, or the trait itself 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/v0.39.0/src/lints/trait_missing.ron

Failed in:
  trait scylla::routing::partitioner::Partitioner, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:78
  trait scylla::routing::partitioner::PartitionerHasher, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-f6d37a628c2cef393fe7e13f8b2fc4220c0154fd/09415b9adb445d3b3772cc894263c395b09e4230/scylla/src/routing/partitioner.rs:95

     Summary semver requires new major version: 3 major and 0 minor checks failed
    Finished [  47.848s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.068s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.028s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.817s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.027s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.110s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  22.554s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1