nested math

[u-fischer]

• correctly handle text in nested math
• clean up socket declarations
• add intent = ':system-of-equations' in equation
• remove Lbl inside mtext as there is an intent.

[zauguin]

@u-fischer This still makes some assumptions about the surrounding tagging context. Wouldn't it be cleaner to run \tag_mc_end_push: at the beginning and \tag_mc_begin_pop:n{} at the end?

[u-fischer]

@zauguin the push/pop commands have quite some overhead, so I would avoid them unless there is clear use case (I even suspect that there are probably never needed and that I could map them to the normal mc-command with luatex, I will try that at some time)