On #715 and #12313 some variants of WeakKeyRef data structures are introduced, used in caching for, among other things, uniqueness of parents and coercions. In #12313 comment 162 is some data on how these data structures can be improved.

Apply:

• attachment: trac_13387-combined.patch
• attachment: trac_13387-guard_gc.patch

Depends on #11521
Depends on #12313

CC: @simon-king-jena @jpflori

Component: memleak

Author: Nils Bruin

Reviewer: Simon King

Merged: sage-5.8.beta2

Issue created by migration from https://trac.sagemath.org/ticket/13387