Move chemistry.mac into the core

[sangwinc]

The contributed maxima chemistry.mac is large

https://github.com/maths/moodle-qtype_stack/blob/master/stack/maxima/contrib/chemistry.mac

To reduce latency in questions caused by loading this, we propose to move this functionality into the core. This means the fixed content will be loaded directly by Maxima rather than physically copied into the question variables and hence subjected to the PHP parsing every time the question is used.