provide configurable policy for handling 'nil' in collections

[bpsm]

Suggested policies:

• Forbid nil. This is the current policy. A nil as a value or key in any collection will trigger a NullPointerException.
• Elide nil. Nil elements in sets and lists will be omitted. Map entires with nil key or value will be omitted.
• Always use optional: all elements of all collections will each be wrapped in an Optional.

Bonus points for:

• Use optional when required: values or keys of any given collection may or may not each be wrapped in an Optional, depending on wether this particular collection contains at least one nil or not.