add dictionary definitions to wiki
Add some definitions of technical terms users of this wiki may not be familiar with.
An example might be Predicativity (or equally, Impredicativity) in the context of type systems. A starter definition for those might be:
"When talking about type systems, and ignoring historical context:
- Predicative: type variables only instantiate to monotypes
- Impredicative: type variables may be instantiated to any type"