Expose SchemaInferenceStack.effectiveStatus()

Our current inference does not propagate effective status into EffectiveStatement hierarchy, but rather performs some indeterminate magic with DocumentedNode.WithStatus.

This mechanics is mostly specified in https://tools.ietf.org/html/rfc7950#section-7.21.2 , but the NETMOD WG list needs to be researched as to how parent/child relationships work – i.e. what would be the effective status of 'bar' here:

module foo {
  prefix foo;
  namespace foo;

  container foo {
    status deprecated;
   
    container bar;
  }
}

as logic would dictate it is implied to be deprecated. This is even stronger when 'status obsolete' is in play – such statements should not be implemented, hence having 'status current' underneath them does not make much sense.

The spec says that

   If a definition is "current", it MUST NOT reference a "deprecated" or
   "obsolete" definition within the same module.

   If a definition is "deprecated", it MUST NOT reference an "obsolete"
   definition within the same module.

it could be argued that child statements are referencing their parent – at least for the purposes of:

• having default effective config inherited from parent
• having a schema node identifier, which references parent's argument

Note that the mechanics of adding status statements has similar scalability implications as YANGTOOLS-1271, hence the two issues will probably share some common infrastructure.