Defining named regular expressions using nested let statements.

If I define a named regexp using nested let statements I get an error when trying to use it.

For example:

``` ocaml
let int_lit =
  let digit = [%sedlex.regexp? '0'..'9'] in
  [%sedlex.regexp? Plus digit]
```

Then later on:

``` ocaml
match%sedlex buf with
| int_lit -> ...
```

`Sedlex: unbound regexp int_lit`.
