z_%exists%_intro and z_%exists%_elim

[rbjones]

Though rules for introduction and elimination of universal quantifiers specific to z exist, the corresponding ones for existential quantifiers don't exist.

[RobArthan]

I'll have a look into this. It might be a good test case for transferring fixes and enhancements from pp to pp-utf8 (or pp2020, should I say?).