Implement DROP COLUMN operation in ALTER TABLE statement

[ivanceras]

This will fix issue https://github.com/andygrove/sqlparser-rs/issues/147

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This change is 

[benesch]

Thanks, this looks good to me! Could you revise so it passes CI? Looks like a few clippy lints failed.

[nickolay]

ALTER TABLE ADD COLUMN was implemented in #203, so I cherry-picked the bits of this PR related to the DROP COLUMN implementation and updated it to close this off.

[coveralls]

Pull Request Test Coverage Report for Build 137649660

• 24 of 28 (85.71%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 91.803%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	5	6	83.33%
src/ast/ddl.rs	6	9	66.67%

Files with Coverage Reduction	New Missed Lines	%
src/ast/ddl.rs	1	83.15%

Totals
Change from base Build 137609215:	-0.02%
Covered Lines:	4144
Relevant Lines:	4514

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💛 - Coveralls

[nickolay]

Thanks!