Support multi reference predigest mode

[maxammann]

I just updated this PR to include a way to have a multi signature pre-digest mode. I think this should be fine. Any pre digest data we found is valid and signed.

Also:

• minor cleanup
• adds comments
• pass through reduce mode in b64 mode

[maxammann]

@njaremko I updated this PR to include a rather major change. Lemme know what you think.

[maxammann]

@njaremko I think I found another issue here. The test in db7039e fails in a certain mode. It returns a response instead of an assertion in the case there is only a single assertion.

[maxammann]

Another thing i noticed is that assertion.signed_info.signature_method.algorithm is no longer populated, where assertion is returned by parse_xml_response_with_mode.

[maxammann]

I think we should:

• remove ValidateAndMark as we can never trust the ancestors. with that mode now we are producing invalid Response ancestors.
• add a way to restrict libxml's selection or allowed signature algorithms: add feature to allow only certain algorithms #89