bug#21915: write inconsistent about #nil

[Zefram]

The write function is inconsistent about whether it distinguishes between
#nil and ():

scheme@(guile-user)> '(#nil . a)
$1 = (#nil . a)
scheme@(guile-user)> '(a . #nil)
$2 = (a)

Thee latter behaviour, emitting #nil as if it were (), breaks the usual
write/read round-tripping, and the traditional correspondence between
equal? and matching of written representation.  Admittedly those standards
are not absolute, nor is the extent to which they're expected to hold
documented, but #nil is clearly sufficiently atomic to be the kind of
value to which one would expect them to apply.  For these reasons,
if a consistent behaviour is to be chosen, I think it should be to
consistently distinguish the values.

I think the behaviour should be consistent.  The values should be
distinguished or not without regard to the context in which they arise
within an s-expression.

Whatever is done, even if it's to endorse the inconsistency, the behaviour
should be documented, with rationale.

-zefram