Antiquote for jump label

Hi, developers of llvm-hs-quote.

I'd like to have a new feature, an antiquote for jump label `$label:`.

The following is my hope.

```hs
-- (THIS IS JUST MY HOPE)
mod :: AST.Name -> AST.Module
mod lab = [Quote.LLVM.llmod|
; ModuleID = 'simple module'
define i32 @foo(i32 %x) {
$label:lab
  %x.addr = alloca i32
  store i32 %x, i32* %x.addr
  ret i32 1001
}
|]
```