Cleaned up atoms

- Atoms now use MFBI to distinguish between thin and owned atoms.
- Introduced TryFromExpr and ToExpr (formerly ToClause) from the old FFI
- Standardized on Bottom being a ProjErr, which means that there will be no RTErr

orchid-api-traits/src/hierarchy.rs

@@ -1,8 +1,3 @@
-/// [Hierarchy] implementation key. The two implementors of this trait are
-/// [Base] and [Subtype]. These types are assigned to [InHierarchy::Role] to
-/// select the implementation of [Hierarchy].
-pub trait HierarchyRole {}
-
 /// A type-level boolean suitable to select conditional trait implementations.
 /// Implementors are [True] and [False]
 pub trait TLBool {}
@@ -13,17 +8,6 @@ impl TLBool for TLTrue {}
 pub struct TLFalse;
 impl TLBool for TLFalse {}
 
-/// Assign this type to [InHierarchy::Role] and implement [Descendant] to create
-/// a subtype. These types can be upcast to their parent type, conditionally
-/// downcast from it, and selected for [Descendant::Parent] by other types.
-pub struct Subtype;
-impl HierarchyRole for Subtype {}
-/// Assign this type to [InHierarchy::Role] to create a base type. These types
-/// are upcast only to themselves, but they can be selected in
-/// [Descendant::Parent].
-pub struct Base;
-impl HierarchyRole for Base {}
-
 /// A type that implements [Hierarchy]. Used to select implementations of traits
 /// on the hierarchy
 pub trait InHierarchy: Clone {