Rewriting op/do macros in Lisp.

The new implementation treats the @1, @2 ... @rest
op arguments as local macros, leveraging the power of the
macro expander to perform the substitution which renames these
to gensyms.

As a result, the implementation is correct.

The old implementation blindly walks the tree
structure doing the substitution, so that @1 is substituted
even though it is in a quoted literal:

  [(op list '(@1)) 42] -> ((#:arg-01-0166))

under the new implementation, '(@1) is left alone:

  [(op list '(@1)) 42] -> ((@1) 42)

* eval.c (expand_quasi): Because the new op macro doesn't
rudely reach into quasi forms to substitute sys:var
elements, relying on macro expansion, we must now macro-expand
sys:var elements. The sys:var macro created by op is
smart enough to skip the compound ones that have modifiers;
they are handled via the inner expansion of the symbol.
That is to say, `@@1` contains the structure
(sys:var (sys:var 1)). The sys:var macro ignores the outer
sys:var. But existing behavior in expand_quasi expands the
inner (sys:var 1), so the substitution takes place.
(eval_init): Do not register the hacky old op and do macros,
except in compatibility mode with 184 or older.

* lisplib.c (op_set_entries, op_instantiate): New functions.
(dlt_register): Register auto-loads for op and do macros via
new functions, except when in compatibility mode with 184
or older, in which case we want the old build-in hacky op
to be used.

* share/txr/stdlib/op.tl: New file.

* txr.1: Fixed or removed no-longer-true text which hints at
special hacks implemented in the op expander. Added
compatibility notes for all new compat-switched op behaviors.

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