deftag: support destructuring of body.

The variable which captures the body part of the tag (material
after attributes) is now a pattern. Thus the body can be
destructured, allowing the custom to effectively have
non-attribute parameters.

* README.md: Documented.

* specials (*cur-form*, *cur-env*): New special variables.

* test/simple.tl: Test case for new deftag feature.

* who.tl (with-html-output): Capture macro invocation's form
and environment, binding them to *cur-form* and *cur-env*.
This is a way of getting this information down to the custom
tag expander, where destructuring errors can occur now.
We would like them diagnosed against the right form.
The body destructuring pattern can use :env too.
(deftag): tag-body parameter renamed to body-pattern. This
is not inserted into the lambda parameter list any more;
a gensym is inserted into that position, and then that value
is destructured using body-pattern.

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