Implementing special variables with local overriding.

All predefined globals become special.

* eval.c (special, with_saved_vars_s): New global variables.
(mark_special, special_p): New functions.
(bindings_helper): Takes new argument, include_specials.
Now processes the special colon syntax for denoting special variables,
setting up their values, taking care to observe whether the
binding is parallel or sequential.
(op_let, op_for): Pass new argument to bindings_helper.
(op_each): Pass new argument to bindings_helper with a value of it,
and deal with the colon annotations that emerge in the bindings.
(op_defvar, regvar): Mark symbol as special.
(op_with_saved_vars): New static function.
(expand_vars): Takes new argument, returns a cons. Detects special
variables among the vars and produces the colon syntax.
(expand_catch_clause): Bugfix: this was using expand_vars on
a parameter list. Now properly uses expand_params.
(expand_save_specials): New static function.
(expand): For the operators that are binding constructs, handle
the new form of expand_vars which returns information about
special variables. If specials occur, then generate the
with-saved-vars form around the expansion which will save and restore
their values. The expansion of vars done by expand_vars, together
with the run-time actions of bindings_helper, do the rest.
Speaking of which, the new with-saved-vars operator form is now
expanded here too.
(eval_init): Protect new variables special and with_saved_vars_s.
Initialize special with new hash table. Store new interned
symbol in with_saved_vars_s. Register op_with_save_vars in op_table.

* txr.1: Documented specials.

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