Add support of accessing the tuple with index field

[git-hulk]

This closes #113

[coveralls]

Pull Request Test Coverage Report for Build 12500586471

Details

• 78 of 104 (75.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 60.351%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/ast_visitor.go	3	5	60.0%
parser/parser_common.go	8	12	66.67%
parser/ast.go	13	23	56.52%
parser/parser_column.go	42	52	80.77%

Totals
Change from base Build 12490522066:	0.1%
Covered Lines:	6396
Relevant Lines:	10598

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💛 - Coveralls