Make MIDIInputMap and MIDIOutputMap more compatible with ES6 Map

Current MIDI*Map has different interfaces from ES6 Map, but function names are same.
I think ES6 iterators are promising, and keep function names reserved for ES6 Map compatible interfaces are reasonable for now.

Before ES6, we can implement forEach() function to be compatible with ES6 map function.
This is only enumerate functions for now, but other interfaces returning iterators will be useful once ES6 iterators and for(let ... of) syntax are ready to use.