interpret: refactor projection handling code

Moves our projection handling code into a common file, and avoids the use of a
general mplace-based fallback function by have more specialized implementations.

mplace_index (and the other slice-related functions) could be more efficient by
copy-pasting the body of operand_index. Or we could do some trait magic to share
the code between them. But for now this is probably fine.

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -529,7 +529,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let receiver_place = loop {
                     match receiver.layout.ty.kind() {
                         ty::Ref(..) | ty::RawPtr(..) => break self.deref_operand(&receiver)?,
-                        ty::Dynamic(..) => break receiver.assert_mem_place(),
+                        ty::Dynamic(..) => break receiver.assert_mem_place(), // no immediate unsized values
                         _ => {
                             // Not there yet, search for the only non-ZST field.
                             let mut non_zst_field = None;