- add BIP header & abstract
- rename MuSig to MuSig2 because some people may want to use the 3-round version
- remove applications because we don't need to motivate an informational BIP
- x-only -> X-only
- remove overly repetetitive "The algorithm [...] is defined as"
- move "Remarks" and "Design" out of "Description" section and move "Test
vectors and ..." into "Description" section. The idea is that the Description
contains everything that is absolutely required to implement the BIP (safely).
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