Functional style Optional and Result return types

[jblachly]

Many functions in the dhtslib namespace may either error, or only optionally return a value.

Paul Backus has an excellent library-based Optional! implementation:
https://code.dlang.org/packages/sumtype

Here is another optional/maybe package:
https://code.dlang.org/packages/optional which we should strongly consider transitioning to as return type.

Also libmir has a different optional implementation (Variant) but looks more complex:
http://mir-core.libmir.org/mir_algebraic.html
Also has a Nullable type