Implement proper conversions between primitives

[bschwind]

When I say "proper", I mean implementing Into, From, TryInto, and TryFrom for things. Every primitive can be converted to a Shape, and if you know the shape type it can also be converted back.

There are also sometimes operations where we want to convert to the base class Shape, do some operation, then convert back to the more specific class we were (such as a Face or Wire). Maybe having a function like self.perform_as_shape(|shape| { }) would be good here, where it converts the inner type to a shape, runs the closure, and then converts back.