Fixes for macOS

Building on macOS with the latest nixpkgs master and NixOS/nixpkgs#77147 fails. It seems some std::experimental (optional) for instance are not available as experimental, but are in std. Also toJSON is missing for atomic< unsigned long long >.

Created by  Hamish Mackenzie  on January 6, 2020
M25EG2BKHICPE5J5OL6T2GKVQOZ5JSYF5LGB3Z7OCUH7RJUHCYGQC
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