1613 Program declarations should be empty after copy

[mhodson-rigetti]

Description

Closes #1613

Back-ported the single-line fix from the v4 branch, so we no longer copy declarations member. Made a new test for Program.copy_everything_except_instructions() which was lacking; based this on the v4 test.

Checklist

• The PR targets the master branch
• The above description motivates these changes.
• The change is atomic and can be described by a single commit (your PR will be squashed on merge).
• All changes to code are covered via unit tests.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.