RO-strings in Guile

[Jim Blandy <jimb@red-bean.com>]

Here's what I want to do: "Efficient substrings via copy-on-write."

    The SUBSTRING primitive should work by sharing space with its
    source string, but both should be marked as shared, so that if
    either is side-effected, Guile will actually copy the string
    first.

    This gets us the semantics of a copy, with the performance of
    sharing.  C code needs to be a little careful, calling a
    SCM_BREAK_SHARING macro before mutating a string, but we've got
    restrictions like this already.  Code review solves this
    problem. 

    We can get rid of read-only strings.

    We can get rid of the MAKE-SHARED-SUBSTRING primitive.

    This will need to interact nicely with threads, but I believe
    there are ways around that.

If we have the above, then we can implement symbol->string using
sharing, making it efficient enough that separate primitives which
accept symbols as strings (like const-string-append) will be
unnecessary.