Add support for MSSQL's SELECT TOP N syntax

[alexkyllo]

Microsoft SQL Server does not use LIMIT syntax to limit the query result set. Instead, the syntax is SELECT TOP (numeric expression) [PERCENT] [WITH TIES]
See: https://docs.microsoft.com/en-us/sql/t-sql/queries/top-transact-sql

This is for Issue #149

This adds a Top child struct to the Select struct. It may or may not be problematic that Select would now have Top nested in it, while the outer Query struct has a limit field, when TOP and LIMIT have basically the same meaning in different SQL dialects. But since TOP is part of the SELECT clause in MSSQL's grammar, this seemed to be the most straightforward way to achieve support for it. Open to suggestions!

[alexkyllo]

I didn't get the clippy::double-must-use error when I ran cargo clippy locally. Trying to reproduce...
Edit: Reproduced, once I updated my rust toolchain. Pushing a commit to remove the redundant #[must_use] attribute

[coveralls]

Pull Request Test Coverage Report for Build 489

• 85 of 87 (97.7%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 93.122%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/query.rs	15	16	93.75%
src/parser.rs	19	20	95.0%

Files with Coverage Reduction	New Missed Lines	%
src/ast/ddl.rs	1	95.4%
src/test_utils.rs	1	91.04%

Totals
Change from base Build 481:	0.2%
Covered Lines:	4644
Relevant Lines:	4987

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💛 - Coveralls

[benesch]

Thanks very much for this! Very appreciated.