See also #36.

This is needed to convert nested alists, when one of the values
is "only" a normal list.

JSON arrays are the default; setting the *LIST-ENCODE* will
produce JSON objects from ALISTs or PLISTs.

Escape control and non-BMP characters as required by the spec.

Fix https://github.com/hanshuebner/yason/issues/43, default to a GENSYM.

Non-BMP characters must be escaped as a utf-16 surrogate pair, and control
characters must be escaped rather than included directly.

Fix specializer for null surrogate value.

When YASON:*PARSE-JSON-NULL-AS-KEYWORD* is non-NIL, the result of parsing
"null" is a keyword, not a symbol in the yason package, so the encoding
method needs to be specialized on the keyword.

Properly skip whitespace when more data is not available (yet)

LISTEN reports false on a stream that is not at EOF, but that doesn't
have data available yet. As-is, reading from a network stream where
the next byte of data isn't available will cause SKIP-WHITESPACE to
stop skipping, which may cause the caller to read a whitespace
character later and become confused.

Rather than checking for available input with LISTEN, instead use
PEEK-CHAR's facility to return a distinguished EOF value and check for
that. When the next byte of data has not been transmitted yet,
PEEK-CHAR will block until it's available and continue skipping
whitespace if appropriate. EOF is still left for the caller to
consume.

Fix #28 Add ENCODE-OBJECT-SLOTS

 Avoid using e notation