Separate instances support for LaTeX environment and respective tests

[jemmybutton]

This adds the support for several named MetaPost instances in LaTeX mplibcode environment. Respective tests are added to test-luamplib-latex.tex.

Syntax is like so:

\begin{mplibcode}[instanceName]
% some mp code
\end{mplibcode}

Behaviour is as follows.

• All the variables and functions are shared only among all the environments belonging to the same instance.
• \mplibcodeinherit only affects environments with no instance name set (since if a name is set, the code is intended to be reused at some point).
• btex ... etex labels still exist separately and require \mplibglobaltextext
• When an instance names is set, respective \currentmpinstancename is set