* Reduce Nim 1.6 compiler warnings/hints for Fluffy and Nimbus proxy
Mostly raises Defect removals, TaintedString removal and some
unnecessary imports.
Also updating the copyright years alongside.
* Further reduce Nim 1.6 compiler warnings/hints for Nimbus
* Move BlockHeaderWithProof content to content key selector 0
- Remove as content type with content key selector 4
- Replace regular block header with BlockHeaderWithProof at
content key selector 0
* Apply blockHeader content key also to bridge
* Add tests for header with proof generation and verification
Kim De Mey