Support multiple WINDOW definitions; expand window function tests

[ajruckman]

Updates WindowClause to hold multiple WindowDefinitions and adjust String()/visitor accordingly.
Now supports comma-separated windows.

Also adds more rigorous tests for window functions.

[coveralls]

Pull Request Test Coverage Report for Build 18914655526

Details

• 122 of 220 (55.45%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 50.557%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/parser_table.go	0	1	0.0%
parser/ast_visitor.go	0	5	0.0%
parser/walk.go	4	9	44.44%
parser/parser_query.go	93	134	69.4%
parser/ast.go	25	71	35.21%

Files with Coverage Reduction	New Missed Lines	%
parser/parser_query.go	1	72.83%

Totals
Change from base Build 18894661377:	0.3%
Covered Lines:	7625
Relevant Lines:	15082

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💛 - Coveralls

[git-hulk]

@ajruckman Thanks for your great enhancement.