Use ImmutableMap for single namespaces

Use ImmutableMap for single namespaces

It is extremely common for statement contexts to have exactly one
namespace (i.e. ChildSchemaNodeNamespace). Tracking that in a HashMap
is not very memory-efficient (nor performance-efficient), as we will
end up allocating all the HashMap internals.

Separate out the single-namespace case and use an ImmutableMap singleton
to hold the namespace -- which saves ~65MiB in a reference test case
accounding for ~6% of scratch memory used (~3% total used).

JIRA: YANGTOOLS-652
Change-Id: Ie2b1faeb4c966c20be0818d3b54aa7c8367f0c58
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>