"Syntactic sugar" for user-defined function calls

[trueqbit]

It is possible to define the expression of a user-defined function call as a variable, which improves readability and "feels" like calling an SQL function.

E.g.

struct IdFunc {
    int operator()(int arg) { return arg; }
};
constexpr auto id_func = func<IdFunc>;

// nice:
select(id_func(42));
// instead of:
select(func<IdFunc>(42));

Also in this PR:

• Fixed a potential crash at the point of calling a user-defined scalar function from the SQLite callback.
• Fixed a potential crash at the point of calling the finalizer of a user-defined aggregate function from the SQLite callback.
• Fixed a memory leak that occurred everytime when deleting or cleaning up user-defined functions.
• Improved naming of variables related to the handling of user-defined functions.
• Delete user-defined functions only after successfully hitting sqlite3.
• Corrected requirement of storage_t<>.count<>().
• Improved compilation times under C++14.