1.2.0

Front End

• VectorTile now holds a lazy Map internally, instead of a Vector. Use
  the ix Lens, giving it a layer name, to hone in on individual Layers
  quickly.

• Users can optionally use the new fromProtobuf function directly from
  Geography.VectorTile.Protobuf if they wish to be semantically explicit
  about the backend conversion. Otherwise, they can still use the top-level
  tile function.

  Back End

• Reworked internals for Protobuf conversion. Most code moved to an
  Internal module.