Achieved by adding a new element property called CarriesTypeIn, whose bits signal to save loading code which properties of particles of the element class in question carry element IDs. The bits in this property are numbered the same way as sim.FIELD_* constants for consistency. One would signal from Lua that a custom element carries element IDs in its tmp like this:
elem.property(id, "CarriesTypeIn", 2 ^ sim.FIELD_TMP)
"Carrying an element ID in a property" is to be interpreted as follows: the property is treated as a combination of a PMAPBITS-bit (so, currently 9-bit) unsigned integer lower part holding an element ID and a 32-PMAPBITS-bit (so, currently 23-bit) signed integer upper part holding whatever makes sense for the element. CONV, for example, uses this signed integer in its ctype as the extra "v" parameter for particle creation.
Used in create_part, kill_part, and part_change_type, allows us to remove element-specific stuff in those functions
Note: difference between Create and ChangeType is that Create is called when we want to initialize default element properties, but ChangeType is called every time a particle is changed to that type, even if it doesn't need default properties set. ChangeType is mainly used for things that need accurate state tracking, like the stkm spawn status.
ChangeType is called every time a particle is changed to or from its type, which includes if the particle is deleted, and also Lua.
Neither of these functions are called when loading saves, that's probably an oversight, will fix later
The rest will will in a commit soon that adds the Create event and some other stuff
Lua events will come after that
PLSM default temp lowered to MAX_TEMP
