Replace cursors with zippers

[ognen]

Internally, replace the usage of cursors with zippers.

In the public facing API:

• Element components receive a new property location which is the current zipper
• The element property is no longer a cursor
• uiForElement and uiForElements are removed
• uiFor accepts the following things as the parameter
  • a string or an array of strings which is treated as now (a key path)
  • a zipper movement function which is interpreted as instruction on how to move from the current location to the one for which we require the UI
  • a selector (zip.select) which is interpreted as an instruction on the selection of elements for which we require an ui for

Given our current top-down architecture, care must be taken so that uiFor does not accept a movement up or sideways from the current location (otherwise shouldComponentUpdate shortcutting won't work).

Consider rewriting the updates reducer more elegantly.

[ognen]

Depends on #53 for the selector feature.

[andon]

Consider rewriting the updates reducer more elegantly.

🤔 I wonder what this means...