Delegate explicit conversions to nil instead of defining them explicitly

This refactoring:

1. reduces the number of lines in `DefineExplicitConversions#call` (`11.downto(6)`),
2. removes 7 single-line method definitions, which are ugly IMHO (i.e., it’s actually a 25-line method, minified),
3. makes the code easier to modify (if new conversion methods are added to `NilClass` in future Ruby versions),
4. works more consistently across different Ruby versions, and most importantly,
5. is more intention-revealing (do what `nil` does vs. do this explicit thing, which happens to be what `nil` does).

I've also taken this opportunity to alphabetize the method names, for organizational purposes.