[TCLCORE] Experimental Mutability patch

[Alexandre F. <ale...@gm...>]

Hello,

Please find attached an experimental Mutability patch against 8.5.0.

FWIW, I followed Neil's advice in restricting mutations to explicit
commands to avoid bad surprises. Only the syntax and implementation
allow for maximal reuse of existing code through a command prefix
[mutant]:

            set l [mutable [list 1 2 3]]
            set n $l
            set x [lreplace $l 1 1] ;# touches neither $l nor $n
            mutant lreplace $l 1 1 ;# touches both which become {1 3}

The patch includes the following:

       - The [mutable] command making a mutable copy of a given list
       - The [mutant] command prefix enabling mutations in the child list op
       - The internal machinery for making it consistent with EIAS
(list of stringified mutables + mass invalidation on mutation)
       - The [debugobj] command to help analyze what happens

It passes the test suite (that one was easy, since nothing is changed
in the absence of [mutable] object creation).

Remarks:

       - So far I have ony done the List part, not the Dict one. But I
think it sufficient as a proof of concept (especially considering the
probability of rejection ;-)
       - What should be interp-wide is actually process-wide. So don't
use it with threads or slave interps. Same remark as above.
       - As you know, the [lrange] and [lsort] functions don't mutate
anything, even under [mutant] steroids, because it is in their DNA to
be non-violent ;-)

       - I am a newbie with diff options for patches, especially under
msys. The patch contains a new file "tclMutable.c" but diff would not
print its contents even with -N or -P. So I cheated and put a 0-sized
copy in the reference directory. Also, I did not include the obvious
Makefile change of adding tclMutable.o. Sorry for the induced
hand-tweaking. Thanks in advance for a tutorial on patch production.

I know that some (many) of you pinch their nose over this. I only hope
that giving it a try may shed a slightly different light. In
particular, look at the simplicity of the new idioms, and also at the
very small size of the patch (high reuse).

Any hands-on comment *much* appreciated !

-Alex