Parse LIKE patterns as Expr not Value

[andygrove]

Closes #578

This fixes a regression in parsing LIKE patterns that was introduced in #569.

I also made some small improvements in the precedence code by extracting more hard-coded values into consts so we can more easily see the relationship between different operators. I added a link to the Postgres precedence docs for reference as well.

[andygrove]

@ayushdg could you take a look?

[coveralls]

Pull Request Test Coverage Report for Build 2877509204

• 42 of 44 (95.45%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 85.448%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	29	31	93.55%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	83.28%

Totals
Change from base Build 2877505870:	0.01%
Covered Lines:	9530
Relevant Lines:	11153

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💛 - Coveralls

[ayushdg]

Thanks for catching the regressions and updating like patterns to accept expr's

[andygrove]

I'm going to go ahead and merge this based on the approval from @ayushdg since this is blocking a number of issues in DataFusion.

[alamb]

Thanks @andygrove