shared-substrings missing in 1.7

[Roland Orre]

It is always annoying when some function one makes heavy usage of
disappears. make-shared-substring is such a function, which is very
handy for conversions and reading fields from fixed width data base
tables.

In guile 1.6 it was said that explicit shared substrings would
disappear, and be replaced by shared strings internally, which
I interpret so that e.g. (substring ...) would return a shared 
substring, which would preserve the functionalily, but... substrings
are still made by copying in 1.7, and now the tag for shared_substring
is also removed.

In about half of my code I can easily replace make-shared-substring
with normal substring but in the rest the functionality of shared
substrings is essential so I need to reimplement them.

The obvious quick and dirty solution is to implement shared-substrings
as scm_tc7_string. (Of course I then have to keep track of the shared
strings to garb them explicitly)

Does anyone have a better idea?

        Best regards
        Roland Orre