Concurrent loading of generated classes can result in a deadlock

[edalquist]

Data classes have proguard protection which results in a static initializer call to Data.nullOf.
Data.nullOf synchronizes on the static NULL_CACHE field.

When a class is loaded it triggers the call to Data.nullOf which then may trigger additional static initializers of field types. If one of the field types in the dependency tree is loaded concurrently it may already be waiting on Data.nullOf.

The result is a deadlock in class loading.

Related to #24