Improve codegen for sized free.

[mjp41]

For languages like Verona or Rust, the deallocation calls know the
size of the object originally requested. This change optimises that
code path to create a much better fast path.

[mjp41]

#109 motivated this PR.

[mjp41]

With this change the rust_dealloc from #109 becomes:

0000000000000050 <rust_dealloc>:
  50:	48 89 f8             	mov    %rdi,%rax
  53:	64 48 8b 3c 25 00 00 	mov    %fs:0x0,%rdi
  5a:	00 00 
  5c:	48 83 c6 ff          	add    $0xffffffffffffffff,%rsi
  60:	48 83 c2 ff          	add    $0xffffffffffffffff,%rdx
  64:	48 09 f2             	or     %rsi,%rdx
  67:	48 81 fa ff ff 00 00 	cmp    $0xffff,%rdx
  6e:	77 49                	ja     b9 <rust_dealloc+0x69>
  70:	48 89 c6             	mov    %rax,%rsi
  73:	48 81 e6 00 00 00 ff 	and    $0xffffffffff000000,%rsi
  7a:	4c 8b 46 08          	mov    0x8(%rsi),%r8
  7e:	48 83 e2 f8          	and    $0xfffffffffffffff8,%rdx
  82:	48 8b 8a 00 00 00 00 	mov    0x0(%rdx),%rcx
  89:	48 8d 97 80 0c 00 00 	lea    0xc80(%rdi),%rdx
  90:	49 39 d0             	cmp    %rdx,%r8
  93:	75 30                	jne    c5 <rust_dealloc+0x75>
  95:	ba 10 08 00 00       	mov    $0x810,%edx
  9a:	c4 e2 68 f7 d0       	bextr  %edx,%eax,%edx
  9f:	48 c1 e2 04          	shl    $0x4,%rdx
  a3:	66 83 44 16 60 ff    	addw   $0xffff,0x60(%rsi,%rdx,1)
  a9:	74 25                	je     d0 <rust_dealloc+0x80>
  ab:	48 8b 4c 16 58       	mov    0x58(%rsi,%rdx,1),%rcx
  b0:	48 89 44 16 58       	mov    %rax,0x58(%rsi,%rdx,1)
  b5:	48 89 08             	mov    %rcx,(%rax)
  b8:	c3                   	retq   
  b9:	48 83 c2 01          	add    $0x1,%rdx
  bd:	48 89 c6             	mov    %rax,%rsi
  c0:	e9 00 00 00 00       	jmpq   c5 <rust_dealloc+0x75>
  c5:	4c 89 c6             	mov    %r8,%rsi
  c8:	48 89 c2             	mov    %rax,%rdx
  cb:	e9 00 00 00 00       	jmpq   d0 <rust_dealloc+0x80>
  d0:	48 89 c2             	mov    %rax,%rdx
  d3:	e9 00 00 00 00       	jmpq   d8 <rust_dealloc+0x88>
  d8:	0f 1f 84 00 00 00 00 	nopl   0x0(%rax,%rax,1)
  df:	00