Enhanced container operations

[deadlocklogic]

Sometimes a container type may have missing semantics in order to be customized as in containers.
For example:

• a sequence container type missing max_size() will fail to compile.

  sol2/include/sol/stack_get_unqualified.hpp

  Lines 399 to 402 in 2b0d2fe

  	for (lua_Integer i = 0;; i += lua_size<V>::value, lua_pop(L, lua_size<V>::value)) {
  	if (idx >= cont.max_size()) {
  	// see above comment
  	goto done;

• a sequence container type missing value_type will fail to compile.
  

  sol2/include/sol/stack_get_unqualified.hpp

  Lines 585 to 587 in 2b0d2fe

  	typedef typename Tu::value_type V;
  	unqualified_getter<as_table_t<T>> g {};
  	return g.get(types<nested<V>>(), L, index, tracking);

Here is a concrete example (actually I encountered a similar case in a library: the containers in the skia library SkSpan, TArray, STArray):

template <typename T>
class Vector
{
private:
    std::vector<T> data; // changed to map

public:
    // Intentionally omitted
    // using value_type = typename std::vector<T>::value_type;
    using iterator = typename std::vector<T>::iterator;
    using size_type = typename std::vector<T>::size_type;
    using reference = typename std::vector<T>::reference;

    iterator begin()
    {
        return iterator(data.begin());
    }
    iterator end()
    {
        return iterator(data.end());
    }
    reference operator[](size_type n)
    {
        return data[n];
    }
};

I suggest deferring such missing traits in the container type to the customization traits sol::usertype_container.
Thanks.