Port substitute_node_with_subgraph to core.

[kevinhartman]

Adds a new graph extension trait substitute_node_with_graph to rustworkx-core, which is currently implemented for StableGraph. It should be easy to open this up to other graph types (which I may consider doing as part of this PR, time permitting).

Todo:

• Migrate Py(Di|)Graph::substitute_node_with_subgraph to use this method.
• Enable for more graph types.
• Release note.

Note to self:

• Check other graph for cycles / multiple edges in Py(Di|)Graph before execution when migrating impl to core method.
• Set node_removed to true.

[coveralls]

Pull Request Test Coverage Report for Build 10890612928

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 156 of 176 (88.64%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 95.753%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	5	6	83.33%
rustworkx-core/src/graph_ext/substitution.rs	111	130	85.38%

Totals
Change from base Build 10152100235:	-0.06%
Covered Lines:	18082
Relevant Lines:	18884

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💛 - Coveralls