Just like with passphrase API, introduce structures, serializers and
parsers for header used by master key API. It's mostly identical but
lacks KDF context.

Make it read like the current passphrase API using the same parsing
techniques. This resolves a bunch of possible vulnerabilities found
by AFL fuzzer which could be triggered by specific corruptions in
the header.

We reuse some utilities from passphrase API files so move them into
common internal header.

Note how we treat the compatibility path. Themis 0.9.6 used to use
incompatble format for one of the context fields used for Soter KDF
which resulted in encrypted data that cannot be decrypted by modern
implementation. We still support decryption of these 'broken' cells
by trying compatibility KDF if the initial decryption fails.

Similarly to decryption, unify the code with passphrase implementation.

Since we use new definitions now, those are no longer necessary.

We have added a new accessor to KDF in Soter algorithm ID, use it.

Just like with passphrase API, we need to pay close attention to the key
length field which we use to determine the length of the derived key.
Accept only formats currently produced by Themis. It's unlikely that we
update them, and Soter cannot handle other formats anyway right now.

Similar to passphrase API, master key header can encode messages with
total length exceeding UINT32_MAX which can cause overflows on 32-bit
platforms (with 32-bit size_t).

Avoid overflows in computations by judiciously using uint64_t and
breaking up intermediate computations to ensure uint64_t is used.

Changes in master key implementations unify error reporting with
passphrase API as well. In particular, we consistently report detected
data corruption with THEMIS_FAIL error code now.

THEMIS_INVALID_PARAMETER is used to indicate programming errors which
could have been prevented by a better type system. Attempting to decrypt
corrupted data is not such an issue.

Some tests that verify corrupted data processing expect particular error
codes to be reported. Update them to expect THEMIS_FAIL now.

We still need to maintain compatibility KDF implementation in order to
be able to decrypt data encrypted by Themis 0.9.6 which used incorrect
Soter KDF context computation, incompatible with 32-bit platforms.

Refactor the compatibility shim code to extact the context computation
and avoid duplicating the entire KDF processing, with all its length and
algorithm verification.