Fix parentheses behaviour

[asdine]

This PR changes how parentheses were parsed by the parser. Previously, they were considered as lists of expressions, which after evaluation turned into an array. This behaviour prevented us from using parentheses as arithmetic sub expressions.

Before:

genji> select ((10 + 5 + 5) * 4) / 2;
{
  "((10 + 5 + 5) * 4) / 1": null
}
genji> select ((10 + 5 + 5) * 4);
{
  "((10 + 5 + 5) * 4)": [
    null
  ]
}

After:

genji> select ((10 + 5 + 5) * 4) / 2;
{
  "((10 + 5 + 5) * 4) / 1": 40
}
genji> select ((10 + 5 + 5) * 4);
{
  "((10 + 5 + 5) * 4)": 80
}

[yaziine]

Only one question otherwise it looks good 👍

[yaziine]

What does that mean exactly?

[asdine]

In SQL, operators are ordered by precedence, for example, * has higher precedence than +, which means it must be evaluated first:

3 + 4 * 2
# 4 * 2 is evaluated first
3 + 8
# then + is evaluated
11

If you want to avoid that, you can use parentheses

(3 + 4) * 2
# 3 + 4 is evaluated first
7 * 2
# then * is evaluated
14

This behavior is done in the parser, when parsing the expression: https://github.com/genjidb/genji/blob/454cc983db979394c310a5a5df0e1180fb0f1447/sql/parser/expr.go#L30-L78

This function builds a tree based on operator precedence.
To avoid an expression in parentheses to have its precedence analyzed and taken into account by that function (these two lines) we "hide" it by wrapping it in a type that doesn't implement the Operator interface so that in the eye of these two lines, that expression is unary, not binary.