Support extension-style calling conventions on possibly native JS objects

[vsmenon]

Methods on String, int, double, bool, num, and (because it may be one of the previous) Object need to be generated differently.

E.g., instead of:

str.hashCode;

generate:

core.String.hashCode(str);

@jacob314 @jmesserly We're handling List and other potentially native interfaces differently. Should we try to be consistent? For the most part, the above are sealed types, so it may make sense to be different.

Object is an interesting case. E.g., we could go:

core.Object.hashCode(obj);

and dispatch appropriately. I'll start working on this. Let me know if you're already looking at something similar.

[jmesserly]

dupe of #40 :)