list-builder: rewrite of basic methods.

Rewriting be addition, pending and nconcing methods of
list-builder to avoid loops and rely on lower list processing
functions. This cleans up the semantics and error messages.

Some examples of behavioral changes:

(build (pend "abc") (add #\d)) now returns "abcd",
consistent with (append "abc" '(#\d)).
Previously it returned '(#\d).

(build (add 1) (pend 2) (pend 3)) now produces a "cannot
append to 2" error.
Previously it produced "copy: cannot copy object of type fixnum".

* share/txr/stdlib/build.tl (list-builder add): Don't use
copy-list; rather the idiom for copying a sequence in
preparation for appending to it is (append x nil). This will
blow up nicely if x is an atom other than nil. We use
this trick twice.
(list-builder add*): Simplify with append.
(pend, pend*, ncon): Rewrite.
(ncon*): Use nconc once on a combined argument list: this is
borrowed from the rewritten pend*.

* txr.1: Documentation updated with clarifications,
particularly in the area of the requirements regarding
destructive manipulation and substructure sharing.

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