Champion "type switch" (part of pattern-matching added in C# 7)

[gafter]

• [ ] Proposal added
• [x] Discussed in LDM (2013-10-07, 2015-01-21, 2015-01-28 2015-03-25a and b, 2015-09-01, 2015-10-07, 2016-04-06, 2016-04-12, 2016-05-03, 2016-07-13, 2016-07-15, 2016-08-24, 2016-10-18, 2016-10-25, 2016-11-15, 2016-11-30, 2016-12-07, 2017-01-17)
• [x] Decision in LDM
• [x] Finalized (done, rejected, inactive)
• [ ] Spec'ed

[gafter]

This is a "proposal issue" because we need to add a proposal/spec to the repo.