(WIP)cpp-ethereum#4598 1)Switch to C++17 style byte. 2)Fixed all compiler …

[ghost]

This PR:

1. Switch to C++17 style byte.
2. Fixed all compiler errors in libdevcore directory.

According to a C++ 17 doc,

1. std::byte is not an integer and not a character.
2. std::byte is storage of bits.

So, I have to define all operators for the new byte. It looks a little messy, but I can gradually change all functions so that those crypto functions don’t require int, char and bigint to interact with byte. In this way, the code should be much nicer~

[gumb0]

Please fix CI build failures, for example

In file included from /home/travis/build/ethereum/cpp-ethereum/libdevcore/OverlayDB.cpp:23:0:
/home/travis/build/ethereum/cpp-ethereum/libdevcore/TrieDB.h: In member function ‘void dev::GenericTrieDB<_DB>::iterator::Node::incrementChild()’:
/home/travis/build/ethereum/cpp-ethereum/libdevcore/TrieDB.h:144:83: error: operands to ?: have different types ‘int’ and ‘dev::byte’
    void incrementChild() { child = child == 16 ? 0 : child == 15 ? 17 : (child + 1); }
                                                                                   ^

C:\projects\cpp-ethereum\libdevcore\CommonJS.cpp(73): error C2593: 'operator ==' is ambiguous [C:\projects\cpp-ethereum\build\libdevcore\devcore.vcxproj]

[chamajcpradel]

Sirs. I agree to these changes. kindly do the needful to fix conflicts. I cannot merge branch.