Optimize hash bucket data storage

Old behavior: Hash bucket data pointer is stored in pData and the
actual data is stored either in pDataPtr (for pointer types) or
a separate allocation.

New behavior: Hash bucket data is appended to the bucket struct.
The bucket data pointer thus implicitly becomes
(void *) ((char *) Bucket + sizeof(Bucket)).

Memory impact: In every case the memory needed for the pData pointer
is saved. For non-pointer types the pDataPtr and allocation overhead
is saved additionally. In numbers:
 * 32bit / ptr :  4 bytes (per bucket)
 * 32bit / data: 16 bytes (per bucket)
 * 64bit / ptr :  8 bytes (per bucket)
 * 64bit / data: 32 bytes (per bucket)

Limitations: It is no longer possible to store variable length data
in a hashtable. This feature was never actually used in PHP.

API changes:

 * zend_(hash|symtable)_update_current_key(_ex) now take an
   additional nDataSize element. This is necessary because the
   potential bucket reallocation needs to know the data size.
 * External code making direct use of the Bucket structure must use
   zend_bucket_data(Bucket *) instead of directly accessing the
   pData member of the bucket. This mostly impacts custom comparison
   functions for zend_hash_sort, as that is the only place where
   well-behaving code is forced to directly operate on buckets.