Function collect_generic_instantiations 

pub fn collect_generic_instantiations(
    services: &HashMap<String, ServiceDecl>,
    agents: &HashMap<String, AgentDecl>,
    boundary_type_names: &[String],
    types: &HashMap<String, TypeDecl>,
) -> Vec<GenericInst>

Collect the set of Result<A, B> / Option<A> instantiations used in boundary positions so the emitter can synthesise the specialised helpers. v0.18: an instantiation may also appear in the fields of a boundary record or sum payload (e.g. the bynk surface’s Request.contentType: Option[String]) — the per-type serialisers delegate to the specialised generic helpers, so walk those too.