Remove un-utilized API.

- These can always be added back to the API if necessary, as
  the main compilation steps won't need them. They are:

  eachChild, everyChild, someChild, mapChild, filterChild,
  before, insertBefore, after, insertAfter, siblings

- Refactored append and prepend for the more common case:
  single node insertion and deletion.

- Removed unnecessary comments.

- Added .shift() and .pop() methods, which remove head and
  tail child nodes.