Disconnect NamespaceStmtCtx from Current and StmtContext

The two user-facing contexts expose namespace access, each of which requires resolving a NamespaceAccess – which ends up going to BuildGlobalContext.

This is currently dictated by legacy class hierarchy and APIs, but we should be able to disconnect these three and passing around a NamespaceCtx:

interface NamespaceCtx {
 
    <K, V> @Nullable Map<K, V> namespace(CommonStmtCtx stmt, ParserNamespace<K, V> namespace);

    <K, V> @Nullable V namespaceItem(CommonStmtCtx stmt, ParserNamespace<K, V> namespace, K key);

    <K, V> @Nullable Map<K, V> localNamespacePortion(CommonStmtCtx stmt, ParserNamespace<K, V> namespace);
}

to all methods which are allowed to access namespaces.

The reactor would implement this through BuildGlobalContext, which would mean that AbstractStorageNode would not need accessNamespace(). There are two wrinkles:

1. the need to cast stmt to ReactorStmtCtx – which is essentially fine
2. the amount of API changes – which entails a lot of work