Unserialize reimplemented (#412)

* Unserialize re-implemented

- more strict and robust parser
- better PHP compatibility, especially in many edge cases
- basic reference support (r and R types)
- support for string with escape sequences (S type)
- support for infinity and nan values (d type)

* Fix whitespaces

* Fix code formatting

* If unserialized array has string keys, return object instead for backward compatibility

* Handle serialized multibyte utf-8 strings

* Fix unserialize example

* Rename cache to initCache for better readability

* Handle serialized multibyte utf-8 escaped strings

* Throw better error message when parsing strings fails