Fix #144: Auto coders for nested anon records

[alfonsogarciacaro]

Fix #144: The problem was Fable assigns an empty name for anonymous records, which conflicts with the internal coder map when having multiple anonymous records. Maybe Fable should assign a name to anonymous records same as dotnet F# does, but giving that anonymous records are represented as plain JS objects, for now the simplest solution is to avoid adding anonymous records to the internal map (they cannot be recursive so there's actually no need for it).

[MangelMaxime]

Thank you for investigating.

Feels a bit hacky ahah