Add support for XOF algorithms

[athoelke]

• Add psa_xof_operation_t and its functions.
• Add SHAKE128 and SHAKE256 XOF algorithms.

Fixes #248

[athoelke]

Would value any feedback on the naming of the input/output functions for the XOF operation API. As noted in the TODO in this PR:

Should it be psa_xof_input() and psa_xof_output() (current), or psa_xof_input_bytes() and psa_xof_output_bytes(), or something else?

For comparison:

• Hash operations use update() and finish(), but only have a single output call. update() and output() doesn't seem quite right.
• KDF operations have input_bytes() and output_bytes(), but also have _key() variants so distinguishing bytes vs keys is necessary for KDF.
• PAKE operations has input() and output() for the data transfer protocol steps.

[MarcusJGStreets]

I think I like psa_xof_input() and psa_xof_output().

As a XOF does not accept a key, the _bytes() seems tautologous to me, and therefore I would go for the shorter name.

So I am happy as is - though obviously the todo section needs to be removed.

I like the change to numbering in tens - reminds me of BBC BASIC. :-)

[athoelke]

Rebased after fixing copyright dates in one of the set-up commits.

[athoelke]

I am tempted to revisit the naming convention question.

All existing multi-part operations that take incremental inputs use the verb 'update' to add inputs: hash, MAC, cipher, AEAD. KDF uses the verb 'input' as there are many different inputs, and they are not streamed into the KDF.

For streamed output, KDF does use 'output', but in this case, the behavior of the output functions is the same as we need for XOF.

So, my revised opinion is to use the pair: psa_xof_update() and psa_xof_output(). psa_xof_update() matches expectation for a developer familiar with the existing multi-part operations.

Although this is not 'a matching pair', that statement is true of all of the other streaming input multi-part operations, which use update/finish or update/compare.

[athoelke]

Changed the input function to psa_xof_update() to align with other multi-part operations in the API.