lazy-let & loop

Hi Bruno, Jörg,

The following is an elaboration on
https://sourceforge.net/p/clisp/bugs/667/#2b06

The problem with loop is that under certain circumstances we separate
initializing and binding of a variable.

I wonder if something like "lazy-binding" could help: bind the variable
but not initialize its value. until it is explicitly set, accessing it
reaches up in the environment. This would require something like
`lazy-let` with the following test case:

--8<---------------cut here---------------start------------->8---
(let ((a 1))
  (lazy-let (a b)
    (print a) ; prints 1
    (print b) ; error - unbound variable
    (setq a 10) ; inner binding of a is activated
    (print a) ; prints 10
    (setq b 22)) ; binding of b is activated
  (print b) ; error - unbound variable
  (print a)) ; prints 1
--8<---------------cut here---------------end--------------->8---

while it is easy to avoid `lazy-let` above by putting `let` after the
second `print`, it is not so easy with `loop` because we need the
binding to exist around the main `tagbody` but to do initialization in
the preamble inside `tagbody`.

Loop expansion then will loop like this:

--8<---------------cut here---------------start------------->8---
(lazy-let (all the loop local variables thay are now bound by let)
  (tagbody 
     preamble - set all the loop local variables thay are now bound by let
     ...
     )) ; close tagbody and lazy-let
--8<---------------cut here---------------end--------------->8---

BTW, we can merge `lazy-let` and `tagbody` into a single form `taglet`:

--8<---------------cut here---------------start------------->8---
(taglet (variables to lazily bind)
  forms...
  tags...)
--8<---------------cut here---------------end--------------->8---

Alternatively, `lazy-let` can be implemented as something like

--8<---------------cut here---------------start------------->8---
(let ((a (if (boundp 'a) a (sys::%unbound))))
  ...)
--8<---------------cut here---------------end--------------->8---


wdyt? thanks.

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