Follow-ups to #261

[real-or-random]

I have a few nits that can be cleaned up in a separate PR. Your detailed and helpful comments in the main code and tests is very much appreciated.

Nits:

• we may want to add a header to the include file that links to the BIP
• there's still a mention of aggsig_size
• we may want to move aggregate before inc_aggregate
• we should mention expected size of input aggsig array in_aggregate
• "Should be aggsig_len = 32*(n+1)" -> "Must be"

Originally posted by @jonasnick in #261 (review)