[ducky64]

Enforces all params (including in ports, excluding arg-params) in non-abstract blocks have assigned values.

Incidental refactors:

• Footprint params are now assigned
• Refactors core tests which ignored unassigned params
• Adds Block._is_abstract internal API
• Fix param binding type in packed blocks

TODOs:

• Change to deprecation for now
• Refactor initializers to generate assigns
• Enforce absence of params for connected ports
• Multipack _packed_connected_ports deduplication
• DRYify _check_params_assigned
• Assert multipacked inner assignments

Follow-on PRs:

• Refactor to use Block.is_abstract API

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• I agree to dual-license this contribution under both BSD 3-clause and Apache License, Version 2.0.