Champion "slicing" / Range (16.3, Core 3)

Introduce a `..` syntax, such that `x..y` ends up creating a `Range` of `x` and `y`.

- [x] Proposal added:
- https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.cs
- https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/index-range-changes.md
- [x] [Discussed in LDM](https://github.com/dotnet/csharplang/blob/master/meetings/2016/LDM-2016-11-01.md)
- [ ] Decision in LDM
- [ ] Finalized (done, rejected, inactive)
- [ ] Spec'ed

See also
- https://github.com/dotnet/roslyn/issues/120
- https://github.com/dotnet/csharplang/blob/master/meetings/2016/LDM-2016-11-01.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-07-05.md#slicing
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-12-06.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-14.md#index-and-range
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-09-05.md#index-operator
- discussion https://github.com/dotnet/csharplang/issues/198
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-09-05.md#index-operator
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-25.md#pattern-based-indexing-with-index-and-range (Index/Range patterns)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-01.md#index-and-range-patterns (Index/Range patterns)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-15.md