Gibber fixes/refactor, mutantrace refactor, couple of small fixes.

Intended changelog:
- The gibber now must be emagged before it will process a human. 

Actual changelog:
- The gibber now must be emagged before it will process a human. It has also been refactored to use existing vars where possible instead of snowflakes.
- Gibbing a human will now result in their internal organs flying out as well.
- Knives and cleavers will now behave more sanely. Large animals will gib and small animals will simply del. Both options will spawn half of the result of using the gibber.
- The various gib procs have been collapsed into a single gibs() proc that accepts a type argument.
- Species flag NO_SLIP is now properly taken into account for wet floors, banana peels, etc.
- A refactor of slip code is included in this. /mob/living/proc/slip() is now a thing.
- The dna.mutantrace var has been entirely removed. This may have broken monkey>human genetics, haven't tested that.
- CAN_JOIN and IS_RESTRICTED species flags have been added. CAN_JOIN allows a species to join from the lobby, IS_RESTRICTED prevents a species from showing up in the chargen preview.
- Shadows, golems and slimes are now restricted species definitions.
- Related to this, species var light_dam has been added. Light over this level causes damage, light below this level heals.
- Species files have been broken up and moved into a better directory structure. Documentation has been updated for species vars.
- makeSkeleton() is now ChangeToSkeleton() to go alongside ChangeToHusk().
- All the terrible golem items have been removed.

Testing:
- [x] Test foam slipping.
- [x] Test soap.
- [x] Test banana peels.
- [x] Test wet and lubed floors.
- [x] Test slime slipping.
- [x] Test golem rune.
- [x] Test slime extract.
- [x] Test animal gibbing.
- [x] Test human gibbing.
- [x] Test robot gibbing.
- [x] Test kitchen gibber.
- [x] Test knife/cleaver on simple_animal mobs.
- [x] Test shadow light damage.
- [x] Test slime person glomp attack.

Issues:
- Icons for a golem and an adamantine golem both exist, couldn't work out where golem is used in the code, so now adamantine is the only option.
- Species icons will reproduce the original sprite faithfully, but my chop work was very utilitarian. Someone might want to fancy them up sometime.
- Semale icons for slimes exist but I only converted over the male. Someone else can do that tedious shit. Slimes will also need a better husk overlay.
- Slimes, golems and shadows will suffer from the reagent processing bugs that currently screw with dionaea due to a lack of kidneys and a liver.

Outside of scope:
- Shadow people and slime people drop limb items when severed or gibbed. There should be either a species proc or an arbitrary limb system to allow for limbs dissolving or otherwise not leaving an item.
- Slimes and shadows (all races, but these two specifically) behave in all respects like a solid body. They should be able to move through grilles and vents, and attacking them in melee combat should be difficult.
- Slime, shadow and golem balance in general needs looking at. Adamantine golems are meant to be slow, but I am not sure if they actually have slowdown.
Interesting, trying to think who has access to the kitchen and the morgue though.
You sure you didn't mean to put it as "one or the other?"  (req_one_access) ?