Preserve double colon casts (and simplify cast representations)

[jmhain]

It's been a slight annoyance to me that :: casts are converted to CAST()-style casts when rewriting SQL using this crate, so this PR makes it so we preserve the cast syntax used in the AST. We also had three AST nodes for CAST, TRY_CAST, and SAFE_CAST which didn't seem necessary, so I reduced this to just Expr::Cast with a kind field that determines the particular syntax / mode of cast used.

This change seems to be well covered by existing tests, but let me know if you have any suggestions for additional tests I could add here.

[coveralls]

Pull Request Test Coverage Report for Build 8680999989

Details

• 37 of 45 (82.22%) changed or added relevant lines in 5 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 88.061%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	9	11	81.82%
src/ast/mod.rs	10	16	62.5%

Files with Coverage Reduction	New Missed Lines	%
src/parser/mod.rs	5	82.86%

Totals
Change from base Build 8660968190:	-0.03%
Covered Lines:	21273
Relevant Lines:	24157

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💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Thanks @jmhain and @iffyio 🚀