Fixes #53.

Restore order of alist to original JSON string

This will be used eg. for VLIME communication.

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)