Auto merge of #15992 - servo:id-table, r=bholley

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 https://github.com/rust-lang/rust/pull/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
```

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