Most of this commit is pure-move.
After this change:
- kernel/coinstats.h
-> Contains declarations for:
- enum class CoinStatsHashType
- struct CCoinsStats
- GetBogoSize(...)
- TxOutSer(...)
- ComputeUTXOStats(...)
- node/coinstats.h
-> Just GetUTXOStats, which will be removed as we change callers to
directly use the hashing/indexing codepaths in future commits.