adds newValue and oldValue on event objects

[justinbmeyer]

For canjs/canjs#4656

Dispatches events with:

action: "set",
key: prop,
target: map,
newValue: newVal,
oldValue: oldValue,

This enables:

map.on("key", ({newValue}) => {})

type functions.

[phillipskevin]

In general, I think this is good. I commented on a few things that I think could be cleaned up.

Changes asked for were done.