`javaClass.classLoader` in `registerSerializersCore` causes a compiler warning

Every generated Kotlin class contains this snippet:
```kotlin
override fun registerSerializersCore(serializers: ISerializers) {
val classLoader = javaClass.classLoader
```

`javaClass.classLoader` causes a Kotlin compiler warning:
> The resulting type of this 'javaClass' call is Class and not Class. Please use the more clear '::class.java' syntax to avoid confusion

Let's either replace it with `val classLoader = Companion::class.java.classLoader`, or add `@Suppress("JAVA_CLASS_ON_COMPANION")` to generated files.

Currently, this issue blocks compiling projects with generated files if we have enabled warnings-as-errors in Kotlin.