Share top-level code for strings

A string constant in GHC turns into

foo :: String
foo = unpackCString# "the-string'

This is a top-level thunk, and it expands into rather a lot of code like this

.text
	.align 4,0x90
	.long	0
	.long	22
.globl _Foo_zdfTypeableTzuds1_info
_Foo_zdfTypeableTzuds1_info:
.LcvI:
	movl %esi,%eax
	leal -12(%ebp),%ecx
	cmpl 84(%ebx),%ecx
	jb .LcvQ
	addl $8,%edi
	cmpl 92(%ebx),%edi
	ja .LcvS
	movl $_stg_CAF_BLACKHOLE_info,-4(%edi)
	movl 100(%ebx),%ecx
	movl %ecx,0(%edi)
	leal -4(%edi),%ecx
	pushl %ecx
	pushl %eax
	pushl %ebx
	movl %eax,76(%esp)
	call _newCAF
	addl $12,%esp
	testl %eax,%eax
	je .LcvL
	movl $_stg_bh_upd_frame_info,-8(%ebp)
	leal -4(%edi),%eax
	movl %eax,-4(%ebp)
	movl $_cvJ_str,-12(%ebp)
	addl $-12,%ebp
	jmp _ghczmprim_GHCziCString_unpackCStringzh_info
.LcvL:
	movl 64(%esp),%eax
	jmp *(%eax)
.LcvS:
	movl $8,116(%ebx)
.LcvQ:
	movl %eax,%esi
	jmp *-12(%ebx)

That's rather a lot of goop for one thunk! Of course we can share this, by making a 2-word thunk like this:

------------------------------
| TopUnpack_info  |   -------|-----> "the-string"#
------------------------------

where TopUnpack_info is a shared RTS info-table and code that embodies the code fragment above.

This would save useless code bloat for every constant string. (This came up when looking at the code generated by deriving(Typeable).)