Casting functions with different context types should produce error

From https://github.com/Perryvw/TypescriptToLua/pull/307#issuecomment-448616990

In the new functions branch, casting functions with context to ones without and vice versa should produce an error:
```ts
interface Callbacks { callback:(this:void,i:number)=>number }
let callbacks = <Callbacks>{ callback:(i)=>{ return 2*i; } } // Should error
callbacks.callback(2) // arithmetic on nil
```
