Updated mpark::variant to their latest master

Their latest master commit (https://github.com/mpark/variant) is 3c7fc8266bb46046b42c2dc2663f9f505f0cec28 from 9 Jan 2020.
This should probably fix a few compile errors, in particular on the latest VS2019, for people who use eos in C++14 mode.