Kotlin Maps with non-string non-numeric keys should map to Map instead of an Object

Kotlin Maps using keys other than number and string should map to Map instead of object.
```Kotlin
data class Foo(val map: Map<Key, String>)
```
maps to
```Typescript
interface Foo{
    map: { [key: Key]: string };
}
```
which is invalid. It should map to
```Typescript
interface Foo{
    map: Map<Key, string>
}
```