Assign the class meta to klass.$$meta

fixup! 2f9235a059 (Move class’ singleton class building to runtime)
Rename boot_makemeta as boot_core_class_object

This function builds the class object of core classes. This commit also puts some internal renames to clarify the implementation and adds some docs.
Abide to linters (JSHint/JSLint)
Reuse boot_class_alloc inside boot_class

The code was almost identical.