Rewrite PropertyDeclaration::id to help the optimizer.

If I’m reading the release-mode assembly correctly, before this change
PropertyDeclaration::id is implemented with a computed jump:

```assembly
	lea	rcx, [rip + .LJTI117_0]
	movsxd	rax, dword ptr [rcx + 4*rax]
	add	rax, rcx
	jmp	rax
.LBB117_3:
	mov	dword ptr [rdi], 65536
	mov	rax, rdi
	ret
.LBB117_2:
	mov	dword ptr [rdi], 0
	mov	rax, rdi
	ret
.LBB117_4:
	mov	dword ptr [rdi], 131072
	mov	rax, rdi
	ret
.LBB117_6:
	mov	dword ptr [rdi], 262144
	mov	rax, rdi
	ret
.LBB117_7:
	mov	dword ptr [rdi], 327680
	mov	rax, rdi
	ret

; Four similar lines repeated for each of the few hundred variants...
```

With Rust 1.15 (currently used for geckolib)
this doesn’t change significantly.
In Nightly 1.17 however, the compiled code uses a lookup table,
possibly thanks to rust-lang/rust#39456.

```assembly
	movq	(%rsi), %rax
	cmpq	$171, %rax
	jne	.LBB23_1
	addq	$8, %rsi
	movq	%rsi, 8(%rdi)
	movb	$1, %al
	jmp	.LBB23_3
.LBB23_1:
	xorq	$128, %rax
	leaq	.Lswitch.table.6(%rip), %rcx
	movb	(%rax,%rcx), %al
	movb	%al, 1(%rdi)
	xorl	%eax, %eax
.LBB23_3:
	movb	%al, (%rdi)
	movq	%rdi, %rax
	retq
```

components/style/properties/properties.mako.rs

@@ -1094,17 +1094,32 @@ impl PropertyDeclaration {
     /// Given a property declaration, return the property declaration id.
     pub fn id(&self) -> PropertyDeclarationId {
         match *self {
+            PropertyDeclaration::Custom(ref name, _) => {
+                return PropertyDeclarationId::Custom(name)
+            }
+            PropertyDeclaration::CSSWideKeyword(id, _) |
+            PropertyDeclaration::WithVariables(id, _) => {
+                return PropertyDeclarationId::Longhand(id)
+            }
+            _ => {}
+        }
+        let longhand_id = match *self {
             % for property in data.longhands:
                 PropertyDeclaration::${property.camel_case}(..) => {
-                    PropertyDeclarationId::Longhand(LonghandId::${property.camel_case})
+                    LonghandId::${property.camel_case}
                 }
             % endfor
-            PropertyDeclaration::CSSWideKeyword(id, _) => PropertyDeclarationId::Longhand(id),
-            PropertyDeclaration::WithVariables(id, _) => PropertyDeclarationId::Longhand(id),
-            PropertyDeclaration::Custom(ref name, _) => {
-                PropertyDeclarationId::Custom(name)
+            PropertyDeclaration::CSSWideKeyword(..) |
+            PropertyDeclaration::WithVariables(..) |
+            PropertyDeclaration::Custom(..) => {
+                debug_assert!(false, "unreachable");
+                // This value is never used, but having an expression of the same "shape"
+                // as for other variants helps the optimizer compile this `match` expression
+                // to a lookup table.
+                LonghandId::BackgroundColor
             }
-        }
+        };
+        PropertyDeclarationId::Longhand(longhand_id)
     }
 
     fn with_variables_from_shorthand(&self, shorthand: ShorthandId) -> Option< &str> {