Add function version of foreign function declaration

[pschachte]

As a convenience/comfort for users accustomed to thinking of foreign procs as functions, allow a syntax like

`def foreign` *language* *function*`(`*param*, *param*, ...`)` `:` *type*

as equivalent to

`def foreign` *language* *function*`(`*param*, *param*, ..., `result`:*type*`)`

If #467/#468 is implemented, this should be extended in the obvious way.