Generalize extern capture and add resources

[b-studios]

This PR generalizes the capture annotated on extern defs. The following are equivalent:

extern pure def foo(): Unit = ""
extern {} def foo(): Unit = ""

extern def foo(): Unit = ""
extern io def foo(): Unit = ""
extern {io} def foo(): Unit = ""

The PR also adds the new concepts of extern interfaces and extern resources:

extern interface StackLocal
extern resource stack: StackLocal

extern interface Array[T]
extern {stack} def makeArray[T](size: Int): Array[T] at {stack} = ""