New System for CO2 Asphixyation

Too much CO2 now affects /living/carbon/humans gradually instead of longer knocking them out after 12 seconds and killing them after 30.

This was going to be the first part of a change to mob CO2 production intended to make the presence of working scrubbers more relevant, except the only way I could think of doing it was to increase breathing rates, requiring corresponding adjustments to the size of all internals tanks. If I do that I will probably propose it in a separate PR.