* Sema: implement peer type resolution for optionals and null.
 * Rename `Module.optionalType` to `Type.optional`.
 * LLVM backend: re-use anonymous values. This is especially useful when
   isByRef()=true because it means re-using the same generated LLVM globals.
 * LLVM backend: rework the implementation of is_null and is_non_null
   AIR instructions. Generate slightly better LLVM code, and also fix
   the behavior for optionals whose payload type is 0-bit.
 * LLVM backend: improve `cmp` AIR instruction lowering to support
   pointer-like optionals.
 * `Value`: implement support for equality-checking optionals.