Support static final `MethodHandle`s and `MethodType`s

[SirYwell]

Currently, all the analysis is done on method-level. Also analysing fields will improve usabililty, as it's a common pattern to have method handles as (dynamic) constants.

Example of what should be supported:

class Example {
	private static final MethodType TYPE = MethodType.methodType(int.class, String.class);
	private static final MethodHandle HANDLE;
	static {
		HANDLE = MethodHandles.empty(TYPE);
	}
	int get() {
		return (int) HANDLE.invoke("Hello World"); // detect issues here
	}
}

[SirYwell]

#19 is part 1 of this: initializers are analyzed, but assignments in bodies are ignored for now.