Summary: nint i = 1; and nuint i2 = 2;

Shipped in preview in 16.7p1.

• Proposal added (https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/native-integers.md, open issues Open LDM issues with native integers #3259)
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

/cc @KrzysztofCwalina @jaredpar

[jcouv update:] as part of this feature, we should consider what Interlocked overloads the BCL/runtime should provide for those new types.