Support drop sequence statement

[samjay000]

Drop for sequence DROP SEQUENCE [ IF EXISTS ] name [, ...] [ CASCADE | RESTRICT ]

[alamb]

Looks great -- thanks @sam-mmm

[coveralls]

Pull Request Test Coverage Report for Build 3251425509

• 20 of 22 (90.91%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 85.838%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	83.4%

Totals
Change from base Build 3227007003:	0.02%
Covered Lines:	10540
Relevant Lines:	12279

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💛 - Coveralls