Make BitSet.terms private

[sequencer]

Discussed in previous dev-meetings:

1. BitSet.terms should be an internal API
2. BitSet might need have a new binding, supporting bitset operators, we need to use MFC to handle it.(After 3.6)

This PR will avoid user using(abusing) terms and BitSet to avoid further migration burdens.

Contributor Checklist

• [ ] Did you add Scaladoc to every public function/method?
• [ ] Did you add at least one test demonstrating the PR?
• [ ] Did you delete any extraneous printlns/debugging code?
• [ ] Did you specify the type of improvement?
• [ ] Did you add appropriate documentation in docs/src?
• [ ] Did you state the API impact?
• [ ] Did you specify the code generation impact?
• [ ] Did you request a desired merge strategy?
• [ ] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• code refactoring

API Impact

private BitSet.terms.

Backend Code Generation Impact

None

Desired Merge Strategy

• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels?
• [ ] Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• [ ] Did you review?
• [ ] Did you check whether all relevant Contributor checkboxes have been checked?
• [ ] Did you mark as Please Merge?