Copy-on-write strings and mutation-sharing strings

[Marius Vollmer]

Hi,

CVS HEAD now has copy-on-write strings and also mutation-sharing
strings.

Test cases are still missing, so go crazy and try to break them!

(The motivation for doing this now is that they make symbol->string
efficient and we thus don't need an extra set of functions for
converting symbols to C strings.  Of course, they are a nice feature
in general and much of the work needed for cow-strings is useful when
properly supporting Unicode later.)

>From the NEWS:

** There is now support for copy-on-write substrings and
   mutation-sharing substrings.

Two new procedures are related to this: substring/shared and
substring/copy.  See the manual for more information.