split mathics.core.formatter

[mmatera]

This PR splits the mathics.core.formatter module in one piece (of the same name) including the routines associated to boxes_to_format, and a second piece mathics.core.makeboxes with the format_element associated routines.

Also, a new function do_makeboxes (should I have named it eval_makeboxes?) splits the task of applying MakeBoxes rules.

[rocky]

Let's leave this open as a draft. This is probably a better model to try to rewrite things than the current code base.