Add with-float-rounding-mode macro

Describe the feature

Add with-float-rounding-mode macro to wrap around code that is run with the rounding mode set to the value provided.

Is there a prototype?

Here is a very rough sketch that kind of lays out the design. This is untested.

(defmacro with-float-rounding-mode (mode &body)
  `(let ((modes (get-floating-point-modes)))
     (unwind-protect
         (progn
           (set-floating-point-modes :rounding-mode ,mode)
           ,@body)
       (apply 'set-floating-point-modes modes))))

where mode is one of the keywords allowed by set-floating-point-modes for the rounding mode.

Ideally, this macro would diddle the FP control word directly instead of using set-floating-point-modes. Also, we wouldn't completely restore the FP modes but only restore the rounding mode.

Describe the feature in more detail

We already provide with-float-traps-masked and with-float-traps-enabled. It makes sense to have a corresponding with-float-rounding-mode too.