Introduce '_current_module' and '_parent_module' variables

Add two built-in variables that provide access to the names
of the current and the previously instantiated modules.

Having these variables simplifies generation of BOMs and assembly
graphs (e.g. with GraphViz).

[std::stack]

Alternate approach to #457. More (maybe?) by the book than #459 as it allocates separate structure for the stack instead of (ab)using the CPU stack. This might be beneficial if there is to be a need to inspect the module stack.