Adds the following util units and adds them to libbitcoin_util:
- `util/url.cpp` takes `urlDecode` from `httpserver.cpp`
- `util/error.cpp` takes `TransactionErrorString` from
`node/transaction.cpp` and `AmountHighWarn` and `AmountErrMsg` from
`ui_interface.cpp`
~~- `util/fees.cpp` takes `StringForFeeReason` and `FeeModeFromString` from `policy/fees.cpp`~~
~~- `util/rbf.cpp` takes `SignalsOptInRBF` from `policy/rbf.cpp`~~
- 'util/validation.cpp` takes `FormatStateMessage` and `strMessageMagic` from 'validation.cpp`
https://github.com/bitcoin/bitcoin/pull/15638/commits/91a25d1e711bfc0617027eee18b9777ff368d6b9
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Depends on D5994
This is a partial backport of Core PR15638