F# nullness at interop boundaries

F# nullness checking makes .NET reference types that can be null visible as T | null. The compiler reports most problems. The risk is the wrong fix: a fix that compiles and hides a real null. Convert each T | null to an F# type once, at the boundary. Then the rest of the code uses T or T option.

scripts/run.ps1 verifies the null results and diagnostics below. Verify lists the rules that it does not verify.

Turn on the checks

<PropertyGroup>
  <Nullable>enable</Nullable>
  <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>