Accepting arbitrary strings as structure field names

[Rafael Laboissiere]

Would it be acceptable to remove the restriction of structure field names
being valid_identifier?  The benefit of this change is that Octave will have
hash arrays for free, allowing code like this:

    var.("@#$%") = 0;

I do not know the impact of this change for the internals of Octave.  At
least, the command line completion would have to be changed, otherwise
invalid syntax may arise when completing structure field names.

If this is implemented, a new variable named something like
generic_strings_as_field_names_ok could be created, which would default to
the current behavior.

-- 
Rafael