[shape] Basic constant folding.

- Implement a first constant fold for shape.shape_of (more ops coming in subsequent patches)
- Implement the right builder interfaces for ShapeType and other types
- Splits shape.constant into shape.const_size and shape.const_shape which plays better with dyn_cast and building vs one polymorphic op.

Also, fix the RUN line in ops.mlir to properly verify round-tripping.