functional variants of string-set! and string-fill!

[Dan Bornstein <danfuzz@xxxxxxxx>]

To support pure functional programming, it seems to me that there ought to
be two new procedures:

    (string-set string i char) -> string
    (string-fill s char [start end]) -> string

which would be like their "!" brethren, except they return new strings
instead of altering their arguments.

-dan