Enable nullable, fix issues, and set to error #20
Description
In concert with ipfs-shipyard/net-ipfs-core#21 and ipfs-shipyard/net-ipfs-core#19
Nullability is a newer feature of C# that helps reduce bugs in caller code as well as within the library code. It can also help avoid the need for excessive null checks and ArgumentNullException by letting compile-time assertions provide enough of a guarantee on correctness from the caller.
- Enable enable in a global property in Directory.Build.props at the repo root.
- Fix all warnings ("get clean")
- Change the nullable warning to an error ("stay clean")