warn upon pin reuse; closes #548

- `Board#register` is now an object, properly indexed by address.
- `Board#register` is of type `Object<String,Array>`, to potentially store multiple objects against the same address
- `Board#serialize` will retain the same behavior if only one object is used for an address; otherwise it calls `reduce()` for each

I couldn't figure out where to put tests for this. 
If we want to use numeric indices, then we'll have to convert between strings like `A0` and whatever the real number of the pin is, which seems like extra work--I figured an Object would be easier.