Allow Function & Type References to Between Extensions

[vbarua]

Problem

It is difficult to re-use existing functions and types from one extension in another extension as there is no mechanism to reference functions and types outside of the current extension.

Use Cases

• When defining fallback expressions for functions, a fallback expression might wish to use a function that already exists, but in a different extension file.
• A system might wish to add functions or types that depend on types that are defined in a different extension

[vbarua]

sigh I opened this here by reflex, and not the spec repo. Will move it.