Signature payload format v1

[Geal]

updated with the lastest changes from a1a2d59. This should work well now. There's one last decision here: do we update the sealing signature payload or remove sealing entirely?

[divarvel]

This looks really good.

A couple suggestions:

• separate the authority block signature description from subsequent blocks, this will prevent invalid combinations such as having an external signature defined, but no previous signature (this does not change anything to the actual behaviour, it just makes the spec clearer). This would also help describing how to sign attenuation blocks as block n+1, with n >= 0, instead of having a single description for block n with n >= 0 with references to n-1 (which is not defined for n = 0)
• reorder signature elements to put the optional elements at the end
• prefix the external signature input with \0EXTERNAL\0 to avoid having the same input for the regular and external signatures