maint/gen_coll: allow custom restrictions

[hzhou]

Pull Request Description

Allow literal restriction conditions to be specified. When the restriction contains any non alphabet, comma, hyphen, or underscore, assume it is a literal condition string. Usually the condition will contain ==, &&, ||, fcall(...), etc.

[skip warnings]

Author Checklist

• [ ] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
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• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
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[srirajpaul]

@hzhou The commit is correctly generating code with the OR condition required for fallback.

[hzhou]

@srirajpaul Do we still need this PR to be merged?

[srirajpaul]

@hzhou oops. I do not remember the place where it was required. I think we can close it for now and reopen and merge if I encounter similar scenario in future.