slashing: rename DoubleSigning to ConsensusEquivocation

[ptrus]

Extracted some general slashing changes from: #3640 (ADR-0005)

• rename DoubleSigning->ConsensusEquivocation
• change SlashReason type to uint8
• change SlashEscrow method to return slashed amount

[codecov]

Codecov Report

Merging #3646 (dc19eb2) into master (5f6c0bf) will decrease coverage by 0.11%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master    #3646      +/-   ##
==========================================
- Coverage   67.39%   67.27%   -0.12%     
==========================================
  Files         390      390              
  Lines       36676    36678       +2     
==========================================
- Hits        24717    24676      -41     
- Misses       8508     8545      +37     
- Partials     3451     3457       +6     

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/staking/staking.go	53.06% <0.00%> (ø)
go/consensus/tendermint/tests/evidence.go	92.72% <ø> (ø)
...o/consensus/tendermint/apps/staking/state/state.go	57.49% <21.42%> (-0.22%)	⬇️
go/consensus/tendermint/crypto/priv_val.go	44.30% <50.00%> (ø)
go/staking/api/slashing.go	77.77% <66.66%> (ø)
go/consensus/tendermint/apps/staking/slashing.go	73.77% <100.00%> (ø)
go/staking/tests/debug/debug_stake.go	92.45% <100.00%> (ø)
go/staking/tests/tester.go	89.39% <100.00%> (ø)
go/consensus/tendermint/apps/governance/query.go	83.33% <0.00%> (-16.67%)	⬇️
go/consensus/tendermint/api/api.go	74.66% <0.00%> (-10.67%)	⬇️
... and 33 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5f6c0bf...dc19eb2. Read the comment docs.