Refactor Symbol to make _length a standalone field again

JDK-8207359 merged Symbol::_refcount and _length into a single field. We should refactor this to instead pack _refcount and _identity_hash, leaving _length as a standalone field.

_length is used in a lot more places, so not having to call a method to extract the value using shifts results in a code size reduction and potential speed-up. Except for a path only used when dumping CDS archives, _identity_hash is only accessed in one place, and there in conjunction with an access to length.

External tools like async-profiler[1] have been found to depend on Symbol::_length, which broke and required a fix in JDK 12. I haven't found any tools that care about the _identity_hash and _refcount fields, though.

[1] https://github.com/jvm-profiling-tools/async-profiler