Miscellaneous fixes

[bedeho]

• Add convenience header
• Drop redundant types, namely non coincore related types (e.g. DataSizeRepresentation)
• Fix warnings

/Users/bedeho/.conan/data/Common/0.1/joystream/stable/build/fdd1a91b977341514ebf098dd786f41bdb91f066/common-cpp/src/Network.cpp:38:1: warning: control may reach end of non-void
function [-Wreturn-type]
}

/Users/bedeho/.conan/data/Common/0.1/joystream/stable/build/fdd1a91b977341514ebf098dd786f41bdb91f066/common-cpp/src/Base58CheckEncodable.cpp:54:1: warning: control may reach end
of non-void function [-Wreturn-type]
}

/Users/bedeho/.conan/data/Common/0.1/joystream/stable/build/fdd1a91b977341514ebf098dd786f41bdb91f066/common-cpp/src/DataSizeRepresentation.cpp:154:1: warning: control may reach
end of non-void function [-Wreturn-type]
}