Unbox first level of options when serializing JSON

[lihaoyi]

Fixes #528

This PR makes Option[T]s serialize to null or unboxed ts by default. Together with the default configuration of serializeDefaults = false, this allows None fields on case classes to be omitted when serializing case classes. Even with serializeDefaults = true, it still serializes Option[T] as null or unboxed t. Either way is much more in line with standard REST API and JSON schema practices than the current serialization of Option[T] as zero-or-one-element-arrays [] or [t]

	Before	After
None	[]	null (or nothing, if a case class field with serializeDefaults = false
Some(1)	[1]	1
Some(None)	[[]]	[null]
Some(Some(1))	[[1]]	[1]
etc.

Despite the convenience, this does mean that there are certain data structures that do not get round tripped. e.g. a field null: Option[T] would get serialized to null, and deserialized as None. This is a tradeoff, but given the rarity of nulls in Scala codebases, and the intuitive expectations of how Options should behave, it seems a reasonable tradeoff.

This PR does make an effort to support nested options: Some(None) is serialized as [null], while Some(Some(t)) is serialized as [t]. This manual boxing allows nested Options to be preserved during round-trip read/write, rather than being flattened out to a single top-level None. These nested options typically do not appear in REST APIs or JSON schemas, and so the choice to preserve round-trip-ability should not affect compatibility with public APIs

This is a breaking change that will need to go into uPickle 4.x. For backwards compatibility, and for migration purposes, the new serialization format is controlled under a flag optionsAsNulls = true. Users who really need the full round-trip preservation of Scala data structures, or who want to preserve compatibility with existing systems, can create a custom config with optionsAsNulls = false. Given the change in the serialization format, I haven't found a way to make uPickle read both old and new formats during the transition, but users can continue to use uPickle 4.x with optionsAsNulls = false indefinitely if they want to preserve compatibility with the old serialization format

For users that want to enable serializeDefaults = true, we should be able to allow serializeDefaults to be configurable on a per-field basis to allow it to be disabled just for the fields typed as Options, to continue eliding them while serializing other default values. Doing that can be done as a follow up