Accept :length option for index definition

Index :length option works in migrations, but is ignored when loading schema. Underlying problem is that ActiveRecord IndexDefinition has a struct member named :lengths, but this value is dumped as :length and #add_index expects the option to be specified as :length.

I left schema_plus support for specifying this option as :lengths in tact, although I'm not sure if it previously had any effect since it was passed to #add_index as :lengths.