quasiquote: support @,expr hack.

For better or worse, TXR Lisp has a dichotomy of
representation that @<atom> produces sys:var syntax, whereas
@<compound> produces sys:expr. This can cause an issue in
backquoting. Suppose you want to use backquote to generate
sytax like (a @b) where the b comes from a variable.
The problem is that (let ((x 'b)) ^(a @,x)) doesn't do
what you might expect: it produces (sys:expr b) rather
than (sys:var b).

This patch adds a hack into the quasiquote expander which
causes it to generate code to do what you expect.

Old behavior:

  1> (expand '^(a @,x))
  (list 'a (list 'sys:expr x))

New behavior:

  1> (expand '^(a @,x))
  (list 'a (let ((#:g0012 x))
             (if (atom #:g0012)
               (list 'sys:var #:g0012)
               (list 'sys:expr #:g0012))))

In other words, x will be evaluted, and the based on the
type of the object which emerges, either sys:var or
sys:expr syntax is generated.

* eval.c (expand_qquote_rec): Implement the above hack.
We are careful to only do this when this exact shape occurs
in the syntax: (sys:expr (sys:unquote item)).

* tests/010/qquote.tl: New file.

* txr.1: Documented.

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