Provide access to generated bytecode

[raner]

Currently, the bytecode that is generated by Projo RCG is ephemeral, i.e., it is created in memory and turned directly into a Java type (without there ever being a .class file).
However, for some scenarios (e.g., front-end Java to Javascript translation) it is necessary to have an actual class file (even if it not necessarily stored on disk).
Projo should provide a way to access generated bytecode (e.g., as a byte[] or InputStream).