Format System.IntPtr and System.UIntPtr as C# 9 keywords nint and nuint?

[stakx]

This isn't as clear-cut as it may be, since System.IntPtr has traditionally been used in interop scenarios to represent unmanaged pointers outside of unsafe contexts. nint "feels" much closer to a regular integral type than it does to void*, so I'm not sure if we should throw those two types in the same basket as all the other primitive types.