* Modernize, improve, reformat, reorganize, etc.
* Use 'null as the default translation for json `null'. Add keyword
`#:null' arguments to control it, and a `json-null' parameter to
change the default. (Note that there is no ambiguity: symbols are
used in hash keys, and JSON restricts them to always be strings, so
`null' can never be a hash key.)
* Properly decode double \u-escape sequences (as UTF-16 surrogate
pairs).
* Add a keyword option to do more string encoding of all non-ASCII
characters.
* Rename `json->jsexpr' (and other way) to `string->jsexpr'. This is
because (a) after using it for a while I still can't remember which
side is which and a `string' in the name makes it clear, (b) it
follows the similar `xexpr' functions.
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