Daniel Gröber (dxld) authored Jul 08, 2019 and Marge Bot committed Feb 17, 2021

This modifies the lifetime of stackElements such that they stay on the
stack until processing of all child closures is complete. Currently the
stackElement representing a set of child closures will be removed as soon
as processing of the last closure _starts_.

We will use this in a future commit to allow storing information on the
stack which should be accumulated in a bottom-up manner along the closure
parent-child relationship.

Note that the lifetime increase does not apply to 'type == posTypeFresh'
stack elements. This is because they will always be pushed right back onto
the stack as regular stack elements anyways.