Align assignments
Cleanup function definition
Add some documentation to boot_class_meta()
Use more explicit names in runtime API

- rename boot_class_meta to boot_class_object
- rename boot_defclass to boot_class_alloc
Check for .rb_stub when looking for methods
Split method_missing stub helpers in smaller funcs
Enable method_missing on bridged classes

Fixes #593.

It works by adding all bridged classes to method_missing stubs subscribers list.