C++20 features described in Before/After tables ("Tony Tables")

This is an attempt to succinctly describe the main (not all) features of C++20, and to update these docs as features are moved through the committee.

See the list of files in the repo — typically one file per feature (hopefully obvious via the file name). Once C++20 is finalizing, we can put it all together into a single doc.

Many of the examples come directly from the original committee proposals (particularly since we started asking for TTs and/or clear examples in the proposals!), so thanks to the proposal authors.

And thanks to eracpp for a number of contributions!

See the C++17 "Tony Tables".

Created with help the support from my employer, Christie Digital Systems, Inc.