Champion "default interface methods" (16.3, Core 3)

- [x] Proposal added: [proposals/csharp-8.0/default-interface-methods.md](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/default-interface-methods.md)
- [X] Discussed in LDM ([2017-03-08](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-03-08.md), [2017-03-21](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-03-21.md), [2017-04-05](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-05.md), [2017-04-18](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-18.md), [2017-04-19](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-19.md#structs-and-default-implementations), [2017-05-31](https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-05-31.md#default-interface-members))
- [X] Decision in LDM (prototype in progress)
- [ ] Finalized (done, rejected, inactive)
- [ ] Spec'ed

Open issues are being tracked at https://github.com/dotnet/csharplang/issues/406

LDM history:
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-03-08.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-03-21.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-05.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-11.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-18.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-04-19.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-05-17.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-05-31.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-06-14.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-17.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-17.md#issues-in-default-interface-methods
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-11-14.md#default-interface-implementations
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-02-25.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-02-27.md#collision-of-lookup-rules-and-decisions-for-base
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-13.md#interface-reabstraction (reabstraction)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-27.md#default-interface-methods
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-01.md#revisit-objectmemberwiseclone
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-29.md#default-interface-implementations-and-base-calls (base calls)