Add Expr::Tuple and parsing support, remove special case in DISTINCT clause

[alamb]

Rationale:

@yuval-illumex is trying to support parenthesized expressions like (a, b) in various places (see #390, and #391)

I believe these are more commonly called tuple or row types in postgres or trino. Rather than adding special case handling for parens in various places, it would be better to simply support parsing tuples in general

Changes

1. Support parsing tuple expressions in select statement
2. Tests for same
3. Remove special case added in Allow parentheses in GROUP BY #390 for distinct clause
4. Add tests in Support parentheses in DISTINCT clause, CURRENT_TIMESTAMP, CURRENT_TIME, and CURRENT_DATE #391 demonstrating this code can parse paren'd expressions in the group by

Reference:

This syntax is supported by postgres:

alamb=# select (1, 'foo');
   row   
---------
 (1,foo)
(1 row)

It is not supported by mysql:

mysql> select (1, 2);
ERROR 1241 (21000): Operand should contain 1 column(s)

[alamb]

FYI @yuval-illumex

[coveralls]

Pull Request Test Coverage Report for Build 1812838772

• 38 of 39 (97.44%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 90.524%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	4	5	80.0%

Totals
Change from base Build 1807150916:	0.04%
Covered Lines:	6897
Relevant Lines:	7619

---

💛 - Coveralls

[alamb]

this was added in #390 but is now covered by the general purpose Tuple parsing

[e-dard]

I took a whizz through this and it made sense to me 👍

[yuval-illumex]

@alamb Much appreciated!!! Looks great :)