feat!: don't assume "main"-function in dataflow + constant folding

[acl-cqc]

Instead, move towards explicitly stating entry-points, in common with remove_dead_funcs and remove_dead_code in #1897 - note this PR follows #1897.

I've also tried to reorganise the code a bit better wrt. prepopulate_df_inputs as we can also handle Conditional and CFG nodes by using existing code, hence renaming to just prepopulate_inputs

BREAKING CHANGE: dataflow analysis on Module-rooted Hugrs will no longer accept input PartialValues for the main function, these should be passed to prepopulate_inputs instead. Also prepopulate_df_inputs is gone, again use prepopulate_inputs. ConstantFoldPass::with_inputs now takes the entry-point node as first parameter as well as the inputs to apply there.

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn 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/v0.39.0/src/lints/inherent_method_missing.ron

Failed in:
Machine::prepopulate_df_inputs, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-passes/src/dataflow/datalog.rs:51

--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/method_parameter_count_changed.ron

Failed in:
hugr_passes::const_fold::ConstantFoldPass::with_inputs now takes 3 parameters instead of 2, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-passes/src/const_fold.rs:75

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.52%. Comparing base (cb7bf75) to head (9f477b0).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           acl/const_fold_dce    #1896      +/-   ##
======================================================
- Coverage               86.54%   86.52%   -0.02%     
======================================================
  Files                     195      195              
  Lines                   35652    35594      -58     
  Branches                32465    32407      -58     
======================================================
- Hits                    30854    30799      -55     
+ Misses                   3011     3009       -2     
+ Partials                 1787     1786       -1     

Flag	Coverage Δ
python	92.34% <ø> (ø)

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[acl-cqc]

This is a very long note to have as a "We could" comment, it'll probably just get out-of-date ;). Perhaps I should just do it now...

[acl-cqc]

Will make this a follow-on to #1901