fix!: Replace LoadFunction::signature with LoadFunction::instantiation

[doug-q]

Closes #1755

This change breaks serialisation. We will already break serialisation with the next release, so we judge this to be acceptable.

BREAKING CHANGE: The LoadFunction::signature field is removed. Replace uses with DataflowOpTrait::signature().

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
      ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.37.0/src/lints/constructible_struct_adds_field.ron

Failed in:
field LoadFunction.instantiation in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/ops/dataflow.rs:350
field LoadFunction.instantiation in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/ops/dataflow.rs:350

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It 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/v0.37.0/src/lints/struct_pub_field_missing.ron

Failed in:
field signature of struct LoadFunction, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/ops/dataflow.rs:350
field signature of struct LoadFunction, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/ops/dataflow.rs:350

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.64%. Comparing base (97ba7f4) to head (0b7f57f).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/ops/dataflow.rs	75.00%	2 Missing ⚠️
hugr-core/src/extension/resolution/types_mut.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1756      +/-   ##
==========================================
+ Coverage   86.28%   86.64%   +0.35%     
==========================================
  Files         179      180       +1     
  Lines       32515    32837     +322     
  Branches    29418    29744     +326     
==========================================
+ Hits        28055    28450     +395     
+ Misses       2760     2751       -9     
+ Partials     1700     1636      -64     

Flag	Coverage Δ
python	92.43% <100.00%> (-0.01%)	⬇️
rust	86.03% <70.00%> (+0.40%)	⬆️

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