Make HashContexts serializable.

* Modify php_hash_ops to contain the algorithm name and
serialize and unserialize methods.

* Implement __serialize and __unserialize magic meth

Make HashContexts serializable.

* Modify php_hash_ops to contain the algorithm name and
serialize and unserialize methods.

* Implement __serialize and __unserialize magic methods on
HashContext.

Note that serialized HashContexts are not necessarily portable
between PHP versions or from architecture to architecture.
(Most are, though Keccak and slow SHA3s are not.)

An exception is thrown when an unsupported serialization is
attempted.

Because of security concerns, HASH_HMAC contexts are not
currently serializable; attempting to serialize one throws
an exception.

Serialization exposes the state of HashContext memory, so ensure
that memory is zeroed before use by allocating it with a new
php_hash_alloc_context function. Performance impact is
negligible.

Some hash internal states have logical pointers into a buffer,
or sponge, that absorbs input provided in bytes rather than
chunks. The unserialize functions for these hash functions
must validate that the logical pointers are all within bounds,
lest future hash operations cause out-of-bounds memory accesses.

* Adler32, CRC32, FNV, joaat: simple state, no buffer positions
* Gost, MD2, SHA3, Snefru, Tiger, Whirlpool: buffer positions
must be validated
* MD4, MD5, SHA1, SHA2, haval, ripemd: buffer positions encoded
bitwise, forced to within bounds on use; no need to validate

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