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.
Also add GameSave::PressureInTmp3 to check for elements with pressure memory and fix TUNG not sampling pressure on creation. This does not in itself fix#822 because tmp3 and tmp4 are still saved in 16 bits each, so full ctypes still don't fit in tmp3.
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
