Implement JSON_PRESERVE_ORDER encoding flag

Implement JSON_PRESERVE_ORDER encoding flag

With this encoding flag, the object key-value pairs in output are in
the same order in which they were first inserted into the object.

To make this possible, a key of an object is now a serial number plus
a string. An object keeps an increasing counter which is used to
assign serial number to the keys. Hashing, comparison and public API
functions were changed to act only on the string part, i.e. the serial
number is ignored everywhere else but in the encoder, where it's used
to order object keys if JSON_PRESERVE_ORDER flag is used.