allow shorter form of pattern fallback in tuple

[nofun97]

This PR allows

let (?: x:42) = (); x

which is a shorter form of

let (x?: x:42) = (); x

Checklist:

• [x] Added related tests
• [ ] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build 408065628647fc534c491502500ab698699860d6-PR-565

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 47.727%

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Files with Coverage Reduction	New Missed Lines	%
rel/value_set_str.go	7	56.7%
<!--	Total:	7		-->

Totals
Change from base Build 8a417ba69868114a04d0925eb17535aff0085e4c:	0.01%
Covered Lines:	4535
Relevant Lines:	9502

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💛 - Coveralls