* 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
- Let accumulator finish its last pre merge epoch (hash_tree_root
on incomplete epoch).
- Adjust code to use isPreMerge and remove isCurrentEpoch
- Split up tests to a set that runs with a mainnet merge block
number and a set that runs with a testing value.