Champion "Nullable reference types" (16.3, Core 3)

- [X] Proposal added: [proposals/nullable-reference-types.md](https://github.com/dotnet/csharplang/blob/master/proposals/nullable-reference-types.md)
- [x] Discussed in LDM
- [ ] Decision in LDM
- [ ] Finalized (done, rejected, inactive)
- [ ] Spec'ed

Note this includes the `object` constraint: `where T : object`.

LDM:
- https://github.com/dotnet/csharplang/issues/790
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-10-04.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-10-11.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-11-08.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-02-21.md (cast)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-02-28.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-03-14.md#nullable
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-03-21.md (`!`)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-04-25.md (generics)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-02.md#nullable-special-members (attributes)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-14.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-30.md (URTANN and flag to turn feature on)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-06-04.md (lambdas, local functions, Conditional methods)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-07-11.md (feature flag, ephemeral oblivious)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-07-16.md (NonNullTypes to opt-in and control warnings)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-08-20.md (`!`, cast, impact of dereference on null-state, extension to nullable value types, `class?`, `MaybeNull`)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-08-22.md#generic-constraints-with-nullable-reference-types
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-09-10.md#nullability-of-constraints-in-overriding
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-01.md (type inference)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-03.md (expressing context)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-10.md#nullable-reference-types-vs-switch-analysis (switch)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-24.md (value types)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-29.md
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-31.md#nullable
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-11-05.md#where-can-you-put-nullable
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-11-28.md#nullable-array-specifiers (nested arrays)
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-12-05.md (lattice)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-01-07.md (OHI, parsing arrays)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-01-09.md#warnings-for-non-nullable-fields-that-are-not-explicitly-initialized (field initializers, syntactic ambiguities)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-02-13.md (ref assignments, nullability of `?.`, `!` on l-values, pure null tests)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-02-20.md (pure null tests)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-04.md#discussion (`default` loophole)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-06.md (pure tests, reachability, anonymous type members, tuple conversions)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-25.md#nullable-reference-types-shipping-annotations (external annotations)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-27.md#nullable-reference-types
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-03.md (ambiguous overrides, dynamic)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-15.md (implied nullable constraints)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-22.md (finally blocks, partials)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-24.md (`MaybeNull`)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-13.md (controlling warnings)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-15.md (attributes)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-07-10.md (`[DoesNotReturn]`)
- https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-07-17.md (events)
- https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-01-06.md (attributes and method bodies, `Task` variance, cast)
- https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-01-08.md (`T??`)
- https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-02-05.md (attributes for dependent calls, pure null tests on unconstrained generic types)