Prenex parametric polymorphism, a.k.a. generics

[anton-trunov]

This would allow user-defined data types that include general operations on maps, like the map with the O(1) length operation, see #471 or the implementation of emptyMap can be moved to the standard library.

inline fun emptyMap<K, V>(): map<K, V> { return null }

Since key and value map types are strict subsets of all types, the right way to do it would be using bounded polymorphism, but at first we can just throw type errors at the time of instantiation.