Change representation of JSON null to symbol null

[johnwcowan]

In the current version of guile-json, JSON null is represented internally as #nil. Because of the magic behavior of #nil, this causes certain problems when trying to discriminate between various internal JSON representations. For example, the programmer will expect (list? j) to detect a JSON object and (null? j) to detect an empty JSON object. But these type discriminators are not correct, because they will also return #t on #nil. So one must write unidiomatic things like (or (pair? j) ((eq? j '())) instead of (list? j).

I suggest (even though it is yet another breaking change) switching to the Scheme symbol null. This is portable, printable and rereadable, and not subject to the above problems. It is also what Racket and the Chicken medea egg (not the json egg) use.

[johnwcowan]

oops

[bjoli]

The racket json library uses a (json-null) parameter for the null value and an optional argument for most procedures that default to (json-null). That seems like a flexible enough way to do it.

[johnwcowan]

Too flexible in my opinion. Settle on one, preferably the symbol null, and stick with it.

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[aconchillo]

There's a bunch of people using guile-json and making this and #33 would be too much of a breaking change, plus I like supporting symbols when building alists for example.

So, with this in mind, I went for the flexible solution suggested by @bjoli in #57. I'd love if you could take a look.