v0.6.0

Added

• An exists filter. field("name").exists() matches the documents which have the field,
  irrespective of its value; field("name").exists().not() matches those which do not.

  A field explicitly set to Value::Null is present and matches. This is the case no existing
  filter could express: eq(Value::Null) and ne(Value::Null) cannot tell a missing field apart
  from one holding null, so "has this document been given a value for this field at all" was not
  answerable.

  The filter reports has_field() == false and so is never elected for an index scan. A missing
  field and a field holding null are stored under the same null key in an index, so an index scan
  could not tell them apart and would disagree with a full scan. get_field_name() still returns
  the name.

  Embedded fields are addressed by their dotted path (field("address.city").exists()), the same
  way Document::contains_field resolves them.