Make it simple to update classes (ie, their prototype) instead of replacing them

[olejorgenb]

Eg. when re-evaluating a class, update its prototype instead of creating a brand new class

[hedning]

Should be pretty doable when evaluating the whole class. Would need logic for two cases:

• class Foo {}
• var Foo = class {}

The var can be useful as classes are let bound by default.