`Into String (Vector Char)` instance creates a vector without a fill pointer

Is there a particular reason to keep the generated `Vector Char` from having a fill pointer, and thus unable to pop? 

```
COALGEX> (coalton (the (Vector Char) (into "wow")))
#(#\w #\o #\w)
COALGEX> (coalton (vec:pop! (the (Vector Char) (into "wow"))))
```
Yields the error
```
#(#\w #\o #\w) is not an array with a fill pointer.
   [Condition of type COMMON-LISP:SIMPLE-TYPE-ERROR]
```