Defines the core types and traits used throughout the facet ecosystem for runtime reflection:

* `Facet`: exposes a `SHAPE` associated const
* `Shape`: The central type that describes the memory layout and capabilities of a type
* Various vtables that define how to manipulate types at runtime
* The `Def` tree, which describes type definitions (structs, enums, etc.)

This crate is foundational to facet's reflection capabilities, providing the type system that enables runtime type manipulation.
