fix #551, implement attribute-set complement in nest

[nofun97]

Fixes #551 .

Changes proposed in this pull request:

• allow {|x, y, z| ...} nest ~|x|a which is equivalent to {|x, y, z| ...} nest |y, z|a

Checklist:

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

[coveralls]

Pull Request Test Coverage Report for Build 650bc54ff8a42eb5940c8478beae4c3d87bcccc9-PR-560

• 12 of 30 (40.0%) changed or added relevant lines in 3 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.07%) to 47.711%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rel/ops_rel.go	10	14	71.43%
rel/expr_nest.go	0	14	0.0%
<!--	Total:	12	30	40.0%	-->

Files with Coverage Reduction	New Missed Lines	%
rel/expr_nest.go	1	0%
rel/value_tuple.go	2	75.2%
rel/value_set_str.go	7	56.7%
<!--	Total:	10		-->

Totals
Change from base Build 0b2eb7a8fca4344311dcd1a85af60c0a82e42763:	-0.07%
Covered Lines:	4533
Relevant Lines:	9501

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💛 - Coveralls