Medical - Change medical to use hashmaps for wound storage

[kymckay]

When merged this pull request will:

• We most frequently want to access wounds by body part, so this makes
  that a constant time lookup.
• The body part index is no longer stored in every wound since it's
  inherent in the wound storage.
• Using body part names as the keys of the hashmap to improve code
  clarity (no more magic numbers).

closes #6468

I've tested these relevant parts of medical which access the wound structures:

• Wound creation still works.
• Menu and action displays still work.
• Damage effect updates (limping).
• Bandaging still works.
• Stitching still works (inc. testing incremental behaviour).
• Wound reopening still works.
• AI treatment still works.
• Litter creation still works.

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[kymckay]

The most awkward part to rework with hashmaps was stitching so give that code particular attention when reviewing. I've tested it pretty extensively though.

[kymckay]

One consideration I just had is that this will break de-serialization across versions. I should probably add some migration code there to convert any array into a hashmap appropriately.

[tbeswick96]

Removed comment, raised in #9244