const (compile-time) functions

[ExpHP]

const functions have been a planned feature since nearly the inception of the project.

This will be a pretty experimental feature, and a lot more stuff is sure to come up once they are worked on. However, here is their status:

• Parse const functions.
• Scope and properly resolve names of const functions.
• Evaluate const functions during const evaluation.
  • we need something new that is embedded into the const evaluation pass. AstVm is not at all suitable for this because it (very deliberately) does not look at context::Defs or context::Consts.
  • cycle detection and caching should probably be extended to function calls (with all of their arguments)
  • add a recursion limit