Make mruby raii cleanup, and fix cleanup bug.

- Functions were cleaned up after mruby, which was wrong.
- Becuase functions could have stored a reference to a Block which refers to mruby.
- And so it would crash on release builds at exit if custom functions were used.

include/bed.h

@@ -14,6 +14,7 @@
 
 namespace bed {
 struct BEd {
+  internal::scripting::RubyState mrb;
   internal::trie::Trie<internal::functions::Function> functions;
   internal::functions::Function no_op;
   internal::functions::Function eof_op;
@@ -24,9 +25,6 @@ struct BEd {
   internal::vase::AppendStorage append{"/tmp"};
   std::unordered_map<std::string, internal::buffer::Buffer *> buffers;
 
-  mrb_state *mrb = nullptr;
-  int arena;
-
   bool help_mode = false;
   bool prompt_mode = true;
   std::function<std::string(BEd &)> prompt = nullptr;