Expansion of port-bonding

[timbernat]

Several refinements and additions planned for the rdutils.amalgamation.bonding module:

• Considering making part of rdutils directly
• Need to test and verify that saturated structures generate (under normal usage) valid RDKit Mols
• Add support for labeling newly-created ports (as commented out here by decrease_bond_order uniquely via int_complement
• Re-implement increase_bond_order to change bond in "one-shot" (rather than by iterating single-order upconversions). This would remove the need to relocate atom IDs by map number after each upconversion and would simplify sanitization
• Canonicalize alternate implementation of _increase_bond_order backend method
• Revisit implementations of dissolve_bond and saturate_ports