change the creature_tracker to use a list of pointers to monsters.

This allows infestation/removal of monsters without changing the references that `Creature_tracker::find` / `game::zombie` returns.
Revert using temporary copy copy of monster during its death function.

The previous commit removed the possible reallocation of the monster object. One can now safely use the monster object until it is actually removed from the creature tracker. So there is no need for the temporary copy.
Adds constructable stone walls.

A really tiny PR, this one is remarkably simple.
Merge pull request #6298 from Rivet-the-Zombie/stone-walls

Adds constructable stone walls.
Revert "Merge pull request #6298 from Rivet-the-Zombie/stone-walls"

This reverts commit 073f4385157d9ef44574eb01e7ce22edbc93a42f, reversing
changes made to cece08cc8511134718ca1557db14534ff4415cf3.

Nice one.
Merge pull request #6300 from KA101/no-terrain-yet

Revert "Merge pull request #6298 from Rivet-the-Zombie/stone-walls"
Merge branch 'BevapDin-creature-tracker'
Merge branch 'master' of https://github.com/CleverRaven/Cataclysm-DDA into conductivityfix
keep in step with upstream master
Minor formatting fix in mutations.json
Merge pull request #6307 from vidarsk/format_json_mutation

Minor formatting fix in mutations.json
Slightly different method of fixing conductivity issue #6190
Merge branch 'master' of https://github.com/CleverRaven/Cataclysm-DDA into conductivityfix
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