add dictionary definitions to wiki

[flippa]

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"