[with patch, positive review] Parent: fixes broken (implicit) invariant between ._element_constructor and self._element_init_pass_parent

[nthiery]

In Parent, there is a (previously implicit) invariant, namely that:

self._element_init_pass_parent == guess_pass_parent(between ._element_constructor)

This invariant was broken when self._element_constructor was set from self._element_constructor_ by call. This made the parent not to be passed properly to _element_constructor.

This patch fixes this (one line), and adds a corresponding doctest. It also includes a related trivial fix to a line that is apparently never used in coerce_maps, and adds a comment about this.

The category patch #5891 depends heavily on this one, as this way of setting _element_constructor becomes the default one for simple parents.

[nthiery]

I should mention that sage -testall passes smoothly with sage-3.4.2-alpha0 (except for a trivial broken test in interfaces.r which also fails before applying the patch). Haven't tried it with sage-3.4.2 final (under compilation)

[mhansen]

Merged in 4.0.1.alpha0.