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.
Makes sense.  I like it, human race best race!