Move mir::Field → abi::FieldIdx

The first PR for https://github.com/rust-lang/compiler-team/issues/606

This is just the move-and-rename, because it's plenty big-and-bitrotty already.  Future PRs will start using `FieldIdx` more broadly, and concomitantly removing `FieldIdx::new`s.

compiler/rustc_codegen_cranelift/src/base.rs

@@ -797,7 +797,7 @@ fn codegen_stmt<'tcx>(
                             let index = fx.bcx.ins().iconst(fx.pointer_type, field_index as i64);
                             variant_dest.place_index(fx, index)
                         } else {
-                            variant_dest.place_field(fx, mir::Field::new(field_index))
+                            variant_dest.place_field(fx, FieldIdx::new(field_index))
                         };
                         to.write_cvalue(fx, operand);
                     }