…reak compilation

In some specific cases, jsii generates some fancy types for unions. For example,
in static functions only, it will generate the following signature:

```java
public static void method(java.util.Map<java.lang.String, ? extends java.lang.Object> arg) {
//                                                        ^^^^ check this out!
}
```

I'm not sure what the value is of doing this; I suspect it doesn't
actually provide any additional typing value.

However, I *do* know that when we combine this with union validation,
which generates the following code:

```java
for (final Entry<String, Object> __item_fc3b34: arg.entrySet()) {
  // ...
}
```

It leads to a compilation failure, because apparently `? extends Object`
is not assignable to `Object` (I feel like it should be, but who am I to
argue with the Java compiler? I'm sure there are good reasons for it to
be this way).

Rather than remove the `? extends Object` type from the signature, which
I'm not sure what it's for, I'm adjusting the type checker to:

```java
for (final Entry<String, ? extends Object> __item_fc3b34: arg.entrySet()) {
  // ...
}