Turn some variants of `Term` into primitives and intrinsics

[kontheocharis]

• TypeOf can be in the prelude
• Cast, Index, Deref can be implemented as intrinsics.
• ArrayTy can be a first-class variant of Term.
• Loop, Return, LoopControl, Assign can be put into Command, which can be a variant of Term
• Decl can become a special variant of a block term statement