MRI-Binding: Enforce UTF-8 strings in 'Marshal::load()' via aux proc

We override 'Marshal::load()' via alias and call it with
a custom auxiliary proc in the alias which sets the encoding
of all demarshalled strings to UTF-8. The case where a user
himself provided a proc to 'load()' is not implemented.

This is definitely a better solution than patching the ruby
source tree. Thanks to github.com/cremno for hints and help!
Remove unused code
Bitmap: Add FIXME