While `toString()` is not really intended as API we should always make sure there is some reasonable string output for debugging purposes.

Signed-off-by: Peter Gafert <[email protected]>

We should unify this and let all domain objects that have a designated source code location implement `HasSourceCodeLocation`.

Signed-off-by: Peter Gafert <[email protected]>

A method taking a `String` as parameter should clearly state what format the string has. A `String` is not an "ASM type".

Signed-off-by: Peter Gafert <[email protected]>

To detect usages of class objects we need to look which classes are referenced in the constant pool. So far the following usage of `SomeClass` would not have been detected by ArchUnit:

```
class Example {
  private Map<Class<?>, Value> map = Map.of(SomeClass.class, someValue);
}
```

In other words, pure usages of `Foo.class` could not be detected, because there was no "access" to `Foo` in the bytecode. However, for each such occurrence the class would actually be present in the constant pool of the referring class. While ASM does not allow direct access to the constant pool, it does allow us to hook into `ldc` instructions, i.e. load constant instructions in the bytecode, that will in turn reference the respective class. The way to detect this is principally an `instanceof` check for `org.objectweb.asm.Type` in `void visitLdcInsn(Object value)`. As far as I could see, any reference of another class as a constant would cause this method to be called with the respective ASM `Type` object.

It would actually be possible to import a lot more constants here. I have looked a little into adding all supported constant types, so it would e.g. be possible to have assertions on `String` values, but then decided to let it go for now. Strings would still be simple, but as soon as `Integer` comes into play (which could also be imported), there are a lot of internal optimizations by the JVM (e.g. `iconst_1`, ...), which makes it hard to do this in a consistent way. I think the most valuable feature by far is to detect constant loads of classes (since those cause dependencies), so I decided to keep it simple for now.

Signed-off-by: Peter Gafert <[email protected]>

Signed-off-by: Peter Gafert <[email protected]>