Minor refactor the column type interface

[git-hulk]

I think the column type desired a dedicated type instead of an expression.

[coveralls]

Pull Request Test Coverage Report for Build 12196740972

Details

• 62 of 83 (74.7%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 60.243%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/ast.go	27	48	56.25%

Files with Coverage Reduction	New Missed Lines	%
parser/ast.go	4	56.3%

Totals
Change from base Build 12081993364:	-0.09%
Covered Lines:	6334
Relevant Lines:	10514

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💛 - Coveralls