Enumerate transitive self types

[eed3si9n]

Fixes #159

Problem

In Scala 3 self type must enumerate the transitive self types,
which in the context of codecs means all the Formats traits for
all the fields in the child trait, not just that of child traits.

trait XFormats { self: YFormats => }

trait YFormats { self: ZFormats => }

trait ZFormats

In Scala 3, the above won't compile complaining

scala> trait XFormats { self: YFormats => }
     | trait YFormats { self: ZFormats => }
     | trait ZFormats
     |
-- Error:
1 |trait XFormats { self: YFormats => }
  |      ^
  |missing requirement: self type YFormats & XFormats of trait XFormats does not conform to self type ZFormats
  |of required trait YFormats

Solution

This genenrally improves the lookup of format type,
first by traversing lazy type, list type, and non-null type,
and looking up the named types in the local schema.
Then recursively all fields' format types are returned to generate
the list of self types.

In general though, since the protocol object that extends all traits
have been working fine in Scala 2, this mostly amounts to
type beurocracy to satisfy Scala 3 (lampepfl/dotty 2214).