Updates for Maybe tests; also adjust explicit/implicit operators

[aggieben]

This is a follow-on to #18. Changes include

• All the maybe tests that should have been in #18
• a few little tweaks to the implementation
• lots of little tweaks and updates to documentation as a result
• Change implicit "out" cast to be explicit instead of implicit, largely due to the guidance found here: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/using-conversion-operators. Importantly, while testing I found that the implicit operator led to unexpected results.