fix!: Don't expose HugrMutInternals

[aborgna-q]

HugrMutInternals is part of the semi-private traits defined in hugr-core.
While most things get re-exported in hugr, we *Internal traits require you to explicitly declare a dependency on the -core package (as we don't want most users to have to interact with them).

For some reason there was a public re-export of the trait in a re-exported module, so it ended up appearing in hugr anyways.

BREAKING CHANGE: Removed public re-export of HugrMutInternals from hugr.

[aborgna-q]

Added to the next breaking-change milestone

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                  Coverage Diff                  @@
##             release-rs-v0.16.0    #2071   +/-   ##
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  Files                         ?      217           
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  Hits                          ?    34435           
  Misses                        ?     5295           
  Partials                      ?     1804           

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[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- 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/v0.40.0/src/lints/trait_missing.ron

Failed in:
trait hugr_core::hugr::rewrite::simple_replace::HugrMutInternals, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/internal.rs:209

--- failure trait_removed_supertrait: supertrait removed or renamed ---

Description:
A supertrait was removed from a trait. Users of the trait can no longer assume it can also be used like its supertrait.
      ref: https://doc.rust-lang.org/reference/items/traits.html#supertraits
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/trait_removed_supertrait.ron

Failed in:
supertrait hugr_core::hugr::rewrite::simple_replace::HugrMutInternals of trait HugrMut in file /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/hugr/hugrmut.rs:22