FulmiCollection 1.6.4

Totally reworked ReflectionUtils class setAccessible methods and derivatives: now they will use functional interfaces to avoid throwing errors in case of failure.
Added Singlet and [NullableSinglet](src/main/java/it/fulminazzo/fulmicollection/structures/NullableSinglet.java] types.
Added StringUtils#decapitalize function to revert StringUtils#capitalize.
Reworked Tuple and [Triple](src/main/java/it/fulminazzo/fulmicollection/structures/Triple.java] objects.
Removed deprecated BiOptional.
Fixed Printable#toString returning possible null value.
Fixed stack overflow error happening for inner classes.
Fixed ReflectionUtils#getClass not recursively obtaining inner classes of inner classes.