Invent a new memory context primitive, MemoryContextSetParent.

Invent a new memory context primitive, MemoryContextSetParent.

This function will be useful for altering the lifespan of a context after
creation (for example, by creating it under a transient context and later
reparenting it to belong to a long-lived context).  It costs almost no new
code, since we can refactor what was there.  Per my proposal of yesterday.