Parameterized externals

[jckarter]

It would be nice to be able to bind a related set of external functions with a single declaration. The mapping to C function names can be calculated in the attribute list, for example:

[T]
external ("sqrt" ++ letterForType("T")) sqrt(x:T) : T;

External definitions could also be parameterized, but would need a mechanism to define for what sets of parameters they should be instantiated.