snowflake: fix rendering of SELECT TOP

[jmhain]

In Snowflake, SELECT TOP only supports an unparenthesized integer constant as its operand. This syntax was being parsed successfully but the Display impl was rendering MSSQL style (i.e. with parentheses), which causes the resultant queries to fail in Snowflake. I fixed this by distinguishing the two different syntactic forms for TOP in the AST.

[coveralls]

Pull Request Test Coverage Report for Build 7293605325

• 20 of 21 (95.24%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 87.678%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	5	6	83.33%

Totals
Change from base Build 7280713756:	0.003%
Covered Lines:	18287
Relevant Lines:	20857

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💛 - Coveralls

[alamb]

Looks good to me -- thank you @jmhain