Memoize the last node in constvalue lists

[Daniel-Cortez]

The root node in constvalue lists is always treated differently than the others, only its next field is used, so it should be safe to repurpose a part of unused space to memoize the address of the last node in the list. This should speed up the addition of new nodes to the end of the list in append_constval().

[Daniel-Cortez]

Tested compiling "modelsizes.inc" (#285)

#include "../include/modelsizes.inc"
#pragma unused MODELS_gColRadius, MODELS_gColOffset, MODELS_gColAABB
main(){}

On my machine the compile time has decreased from ~13.7 to ~0.95 seconds.

[maddinat0r]

Your changes will make the compiler crash if someone in the future decides to actually write something into the name array of a root constvalue node. You should at least add a comment in the code describing the behavior you depend on, and explicitly warn about not writing anything into the root node.

[Daniel-Cortez]

@maddinat0r Well yeah, now that I look at the changes I've made in this PR I think the current implementation is pretty hackish. Just adding comments won't really help, it rather needs a complete rewrite and that's exactly what I'm doing as of writing this.

[Daniel-Cortez]

Ok, so this time instead of just being casted from constvalue to constvalue_root all root nodes are explicitly defined as of type constvalue_root. This way the code should be more readable since now it's easier to distinguish the root nodes from the regular ones.