new XGM gas mixture datum

Gases are now represented in an associatiee list instead of harcoded
 variables.  gas = list("oxygen" = 20, "nitrogen" = 80)
New cleaned up share_ratio(), share_space() and equalise_gases() procs.
Gas can be modified with adjust_gas() or adjust_gas_temp(), with
 variadic versions available for both. These are documented in
 gas_mixture_xgm.dm

Signed-off-by: Mloc-Argent <colmohici@gmail.com>
integrate XGM into the code

New turf proc: assume_gas().  Maps to air.adjust_gas_temp().
Lots of optimizations to processing, fire, lighting, HasEntered() and
 more.
Zones now process all fire data and existance in one go, fire objects
 only handle spreading.
Most code has been ported straight so some of it mightn't be ideally
 structured for the new gas_mixtures.

Signed-off-by: Mloc-Argent <colmohici@gmail.com>