Move `Object#dig` to `Struct#dig`
This method seems to be mistakenly put into `Object` instead of `Struct`
since it's in `struct.rb` and says:
add #dig to Array,Hash and Struct
MODIFY
mrbgems/mruby-struct/mrblib/struct.rb
mrbgems/mruby-struct/mrblib/struct.rb
@@ -81,7 +81,6 @@ if Object.const_defined?(:Struct)
# 15.2.18.4.11(x)
#
alias to_s inspect
- end
##
# call-seq:
@@ -100,4 +99,4 @@ if Object.const_defined?(:Struct)
end
end
end
-
+end

Move `Object#dig` to `Struct#dig`

This method seems to be mistakenly put into `Object` instead of `Struct`
since it's in `struct.rb` and daf83946b says:

  add #dig to Array,Hash and Struct