Provide a way to invoke generic functions with explicit type arguments

[gbrgr]

Feature Request

Description of Problem:

Suppose I want to define a type that models a bijective Map like so:

Class BiMap<K, V> {
  to: Map<K, V>[1];
  from: Map<V, K>[1];
}

Now I want to provide to users a convenient constructor method that just yields an empty instance of BiMap:

function empty<K, V>(): BiMap<K, V>[1] {
  ^BiMap<K,V>(to=^Map<K,V>(), from=^Map<K,V>());
}

Such a function definition does not work for two reasons: First, PURE tries to find existing types named K and V. Second, there is no way to invoke the empty function by explicitly specifying type parameters at invocation (type parameters are always implicit via supplied function arguments).