Don't use match-destructuring for derived ops on structs.

All derive ops currently use match-destructuring to access fields. This
is reasonable for enums, but sub-optimal for structs. E.g.:
```
fn eq(&self, other: &Point) -> bool {
    match *other {
	Self { x: ref __self_1_0, y: ref __self_1_1 } =>
	    match *self {
		Self { x: ref __self_0_0, y: ref __self_0_1 } =>
		    (*__self_0_0) == (*__self_1_0) &&
			(*__self_0_1) == (*__self_1_1),
	    },
    }
}
```
This commit changes derive ops on structs to use field access instead, e.g.:
```
fn eq(&self, other: &Point) -> bool {
    self.x == other.x && self.y == other.y
}
```
This is faster to compile, results in smaller binaries, and is simpler to
generate. Unfortunately, we have to keep the old pattern generating code around
for `repr(packed)` structs because something like `&self.x` (which doesn't show
up in `PartialEq` ops, but does show up in `Debug` and `Hash` ops) isn't
allowed. But this commit at least changes those cases to use let-destructuring
instead of match-destructuring, e.g.:
```
fn hash<__H: ::core:#️⃣:Hasher>(&self, state: &mut __H) -> () {
    {
	let Self(ref __self_0_0) = *self;
	{ ::core:#️⃣:Hash::hash(&(*__self_0_0), state) }
    }
}
```
There are some unnecessary blocks remaining in the generated code, but I
will fix them in a follow-up PR.

compiler/rustc_expand/src/build.rs

@@ -152,6 +152,19 @@ impl<'a> ExtCtxt<'a> {
         ast::Stmt { id: ast::DUMMY_NODE_ID, span: expr.span, kind: ast::StmtKind::Expr(expr) }
     }
 
+    pub fn stmt_let_pat(&self, sp: Span, pat: P<ast::Pat>, ex: P<ast::Expr>) -> ast::Stmt {
+        let local = P(ast::Local {
+            pat,
+            ty: None,
+            id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Init(ex),
+            span: sp,
+            attrs: AttrVec::new(),
+            tokens: None,
+        });
+        self.stmt_local(local, sp)
+    }
+
     pub fn stmt_let(&self, sp: Span, mutbl: bool, ident: Ident, ex: P<ast::Expr>) -> ast::Stmt {
         self.stmt_let_ty(sp, mutbl, ident, None, ex)
     }