Add runtime classes to common module, to avoid conflict

[smarek]

per #4 (comment)

• Update runtime definition
• Update kaitai_struct_compiler LuaCompiler.scala and LuaTranslator.scala to reflect the changes
• Update kaitai_struct_tests LuaSG.scala to match runtime changes

Names of currently used modules, are relatively too common, struct, enum, class, utils and possibly unicode could conflict with different files from user's runtime or project

[robertlzj]

I post a suggestion about library interface entrance and file structure at #18 . 😁