Feature/undo redo

[dxinteractive]

Addresses #277

Parcel's buffer was already made of an array of changes, that get applied to a value on submit. Perfect for exposing this as undo / redo. Care is taken to only store data that says how to make the changes, rather than cached data of the result of those changes, so it should be really quite efficient on memory.

Works with rebasing (so if your "save" action changes your parcel data), works with submitting changes after having undone a few actions. Even works with undoing actions past the point where the data was last submitted and going into negative change territory.

react-dataparcels

• Adds parcel.meta.undo() and parcel.meta.redo() to useParcel, useBuffer and Boundary
• Adds parcel.meta.canUndo, parcel.meta.canRedo & parcel.meta.synced booleans
• useParcel and useBuffer have an optional history param, to set the minimum number of history items to always remember. Often there'll be many items being remembered anyway because the history array is only cleared on submit / debounce. This history param instructs the hooks not clear all history items on submit if it would result in a number of history items lower than history.