Custom builder improvements; enum improvement

• Fix custom builders in null safe code: allow nested builder fields to be
  nullable.
• Improve custom builders for null safe code: allow abstract setter/getter
  pairs instead of fields. This allows nested builders to have a setter that
  accepts null and a getter that guarantees not to return null, which is
  what auto instantiation of nested builders already provides.
• Allow use of super field initialization in EnumClass.