feat: Parse special keywords as functions (current_user, user, etc)

[ovr]

Hello!

Note: current_catalog, current_schema, current_user, session_user, and user have special syntactic status in SQL: they must be called without trailing parentheses. (In PostgreSQL, parentheses can optionally be used with current_schema, but not with the others.)

https://www.postgresql.org/docs/9.1/functions-info.html

Thanks

[coveralls]

Pull Request Test Coverage Report for Build 2839618346

• 39 of 50 (78.0%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 89.945%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	7	10	70.0%
src/parser.rs	5	13	38.46%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	83.21%

Totals
Change from base Build 2839268478:	-0.04%
Covered Lines:	9240
Relevant Lines:	10273

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💛 - Coveralls

[alamb]

Thank you @ovr -- looks good to me except for a question about the special field

[alamb]

I wonder why a special field is needed ? Wouldn't it be enough to match on the name?

If we do need a special field, can you please add a /// comment explaining what it means and what it is used for?

[ovr]

I use this field to skip formating (arguments).

Right now, Dialect is not available in formating, because we use fmt::Display for implementing to_string() method.

[alamb]

Marking this as a draft to show it is waiting some changes so I can easily filter out which PRs need review. Please mark it as "ready for review" when it is next ready.

[alamb]

Thanks @ovr