[plt-scheme] patch: string->xexpr

No sure if this is the right place, but I was looking in vain for
`string->xexpr`. It might be useful to add it to
`collects/xml/private/xexpr.ss`

    (define (string->xexpr str)
      (xml->xexpr (read-xml/element (open-input-string str))))

    (provide/contract
      [string->xexpr (string? . -> . xexpr/c)])

--
troels