API: builder children interface

[mcmikecreations]

Added children interface. It can take any combination of lvalues and rvalues, universal, copy, and move references. This allows for complex tree hierarchies to be created.
parent_get() points to the immediate last child in the hierarchy.
The only downside is that the hierarchy needs to be created bottom-up:

auto classes = b::children(m.heap_type("Rectangle"), m.heap_type("Circle"));
auto ns_empty = m.scope("Empty") << m.none(); // to allow an empty namespace instead of a global one.
auto ns_shapes = m.scope("Shapes") << classes; // root points to "Shapes", parent points to "Circle".
auto factory = m.source_root("Main.cs") << b::children(ns_shapes, ns_empty); // root points to "Main.cs", parent points to "Empty".

// Main.cs
// namespace Shapes {
//     class Rectangle {}
//     class Circle {}
// }
// namespace Empty {}

[SergiyIvan]

Why backwards?

[mcmikecreations]

All children need to be observed before the memory is freed. The memory needs to be freed through the pointer to the first element in the array aka the pointer to the array in general. Thus, iterating backwards is simpler.

[SergiyIvan]

Looks good!