Introduce a non-echoing version of `\WP_CLI::print_value()`
Always represent serialized arrays as JSON

See https://github.com/wp-cli/wp-cli/pull/793/files#r6644163
... unless, of course, the format is JSON because JSON will handle our data just fine

Re f0d004e
Update `\WP_CLI\Utils::format_items()` to be smart enough about formatting a single item
If the format requested is JSON, provide a useful JSON object.

Reduces redundancy of 'meta_key' => 'x', 'meta_value' = 'y'
Fix copy paste error
Meta list functional tests
Support for iterators