Improving parenthesizing of binary expressions

[trueqbit]

The serialization of binary expressions currently suffers from two problems:

1. The statement serializer too eagerly puts parentheses around every built-in function and binary expression (comparison, logic, arithmetic, bitwise operators) and is additionally dependent on a flag provided by the outer context.
2. Subqueries in binary expressions in columns currently only work because the outer expressions do not change the serializer_context<>::use_parentheses=true flag. As soon as it is changed, as in CTEs, the serialization goes wrong.

1. can be improved and 2) remedied in the following ways:

• Binary operators/conditions: Parentheses are only needed to preserve precedence by following simple logic: only "subtree" expressions need to be parenthesized, "leaf" expressions do not.
• No parentheses around built-in functions necessary - they are already independent expressions (and the outer expression already takes over the parentheses, e.g. check(), default()).
• Establishing a parenthesized subcontext for potential subqueries in binary expressions, such that select(&Employee::salary >= select(avg(&Employee::salary))) works.

This leads to a simplified approach, as the serializer is independent of the context.
In addition, fewer characters are serialized with simple binary expressions, which also improves the readability of statements when they are examined.

Also in this PR:

• Allow conditional expressions to be equality-comparable, so expressions like select(c(5) > 3 == true) are possible instead of select(c(5) > 3 == c(true))
• Merged handling of binary conditions and operators (AST iteration, serialization)
• Appveyor vcpkg environmnent updated to 2023.12.12.
• Fixed "left_and_inner_join" example

[fnc12]

@trueqbit everything is brilliant (as always) except two items