feat: Import model in python.

[zrho]

This draft PR contains part of the import process from the model representation to the Python representation.

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum 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/v0.43.0/src/lints/enum_missing.ron

Failed in:
enum hugr_core::hugr::linking::NodeLinkingError, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/linking.rs:118
enum hugr_core::hugr::linking::NodeLinkingDirective, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/linking.rs:141

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant 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/v0.43.0/src/lints/enum_variant_missing.ron

Failed in:
variant BuildError::HugrInsertionError, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder.rs:183
variant BuildError::HugrViewInsertionError, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder.rs:189

--- 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/v0.43.0/src/lints/inherent_method_missing.ron

Failed in:
ModuleBuilder::link_hugr_by_node, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder/module.rs:231
ModuleBuilder::link_view_by_node, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder/module.rs:246

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.43.0/src/lints/module_missing.ron

Failed in:
mod hugr_core::hugr::linking, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/linking.rs:1

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.43.0/src/lints/trait_method_missing.ron

Failed in:
method add_link_hugr_by_node_with_wires of trait Dataflow, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder/build_traits.rs:259
method add_link_view_by_node_with_wires of trait Dataflow, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/builder/build_traits.rs:295

--- 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/v0.43.0/src/lints/trait_missing.ron

Failed in:
trait hugr_core::hugr::linking::HugrLinking, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/linking.rs:18