Add FixedKeyOutValue Semi Dictionary Type

[ColmBhandal]

Requirement

Add a generic dictionary interface as part of the semi dictionary collection that is covariant in the value but has a fixed generic type for the key.

Value Proposition

A common use case for dictionaries is to read them only, not write to them. Reading a dictionary usually involves iterating over its keys and values. This could be done by making both keys and values out types and adding a new function to iterate over pairs. However, using the existing functionality, we need only defined an interface for fixed key type and variant value type. And anyway, the key type is usually something simple, so variance on key types doesn't seem as important.