Improve Nullable<T> conversions & support

[Ilia-Kosenkov]

• Add TryFrom<RObj> and its variations for Nullable<T>
• Add conversions to and from Options<T>
• Add map() method that works similar to that of Options<T>, allowing acting on the state while propagating NULL
• Add a few doctests
• Add integration test
• Fix integration tests setup/config, update with newest versions of {rextendr} and {roxygen2}

[multimeric]

Does this PR relate to #313 in any way?

[Ilia-Kosenkov]

Not directly, but maybe it covers some of the cases listed there. The original issue is about Option<T>, which we reserve for NA.