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!

patches/ruby/marshal_utf8.patch

@@ -1,15 +0,0 @@
-diff --git a/marshal.c b/marshal.c
-index 4cba05d..dfce6ee 100644
---- a/marshal.c
-+++ b/marshal.c
-@@ -1312,7 +1312,9 @@ r_unique(struct load_arg *arg)
- static VALUE
- r_string(struct load_arg *arg)
- {
--    return r_bytes(arg);
-+    VALUE str = r_bytes(arg);
-+    rb_enc_associate(str, rb_utf8_encoding());
-+    return str;
- }
- 
- static VALUE