op: new features for anonymous recursion.

Within the op syntax, the new implicit variable @rec
now refers to the function itself. There is also @(rec ...)
for calling the function through a function binding.
For instance, here is Fibonacci:

(do if (> @1 1) (+ @(rec (pred @1)) @(rec (ppred @1))) 1)

* share/txr/stdlib/op.tl (sys:op-ctx): New slots rec and
recvar.
(sys:op-rec-p, sys:op-ensure-rec): New functions.
(sys:op-alpha-rename): Check for the new syntaxes and
translate to appropriate gensymed expressions, while updating
the context structure, so the expander is informed about
the @rec or @(rec ...) activity in the expression.
(sys:op-expand): Check whether @rec or @(rec ...) has
been used in the expression, and generate the necessary
variants to support it. We need to bind the lambda to a
recursive binding using the same mechanism that labels
uses, and possibly to bind the gensym underneat @rec
to the value of that function binding.

* txr.1: op documentation extended to cover the new
feature, plus some wording improvements.

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