destructive flonum operators

[ramsdell]

There is a need for flonum operators that overwrite existing
flonums. This will reduce the garbage produced by such calculations.
For example,

(let ((a 0.0))
   (fl+! a 3.0)
   (fl+! a 2.3))

should have only one pointer value given to variable a
during the entire execution of the let, but the value final value of a should be 5.0.

John Ramsdell
-------