Use sha512 prehash on signbytes in x/sigs

[ethanfrey]

We need this so we can sign with the ledger (it requires sha256 or sha512 or blake2b prehash before signing).

It shouldn't break anything except the js clients (which Simon updated already), as your go clients, should also use BuildSignBytes directly or indirectly before signing.

I would like to get this in a (breaking) bov release by the next testnet, so we can release the updated js libs with it.

[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 0.2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #87     +/-   ##
=======================================
- Coverage    78.2%    78%   -0.3%     
=======================================
  Files          62     62             
  Lines        2502   2584     +82     
=======================================
+ Hits         1958   2016     +58     
- Misses        433    458     +25     
+ Partials      111    110      -1

Impacted Files	Coverage Δ
x/sigs/controller.go	78.7% <100%> (+0.3%)	⬆️
x/sigs/context.go	16.6% <0%> (-28.8%)	⬇️
x/sigs/errors.go	75% <0%> (-25%)	⬇️
x/validators/errors.go	100% <0%> (ø)	⬆️
x/cash/errors.go	100% <0%> (ø)	⬆️
examples/mycoind/app/examples.go	0% <0%> (ø)	⬆️
examples/mycoind/app/app.go	82.6% <0%> (+0.6%)	⬆️
errors/main.go	97% <0%> (+1.3%)	⬆️
orm/query.go	93.7% <0%> (+1.4%)	⬆️
orm/bucket.go	82.6% <0%> (+2.1%)	⬆️
... and 4 more

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[alpe]

looks good! I have seen this in other contexts as well when a signed container image should be verified for example.
I wonder if you want the hash algorithm as an extension point but that can be done easily whenever needed

[ethanfrey]

I wonder if you want the hash algorithm as an extension point but that can be done easily whenever needed

Yes, that is a good idea. I would love to if there was more time. So the repo extending weave could specify it. I will add this as an issue, but it will be non-breaking for our network, so upgrade can happen without client noticing.