Uniform syntax for column pointer expressions through table references

[trueqbit]

Since the advent of a new syntax for alias columns in the form of

constexpr auto z_alias = "z"_alias.for_<Object>();
select(z_alias->*&Object::id);

and the upcoming CTE feature that uses the same notation:

constexpr auto xaxis = "xaxis"_cte;
select(xaxis->*x);

it would be very convenient and again a huge improvement to the legibility of SQL statement expressions if column pointers can be formed using the same syntax.

struct Object { int64 id; };
struct Derived : Object {};
constexpr auto derived = c<Derived>();
select(derived->*&Derived::id);

Additionally, this PR contains the following improvements:

• A lot more expressions are now available for column pointers (e.g. storage.count(column<Derived>(&Derived::id)) works)
• A myriad more unit tests for column pointers
• More unit tests for alias column pointers

It must be clarified how a "table reference" can be created. Currently available:

// 1.
constexpr auto derived = c<Derived>();
// 2.
constexpr auto derived = column<Derived>();

[fnc12]

thanks @trueqbit