Add ImmutableMapTemplate

Add ImmutableMapTemplate

We have couple some hot paths where we know the keySet of an immutable
map we want to instantiate. For these, using
ImmutableOffsetMap.*copyOf() is wasteful, as it performs unnecessary
cache looks and map lookups.

ImmutableMapTemplate can be used to amortize these preparatory steps,
so that an ImmutableOffsetMap (or SharedSingletonMap) can efficiently
be produced by either transforming values of some other map, or by
supplying values in the correct order.

Since we are in the area, improve documentation and @NonNull
annotations of both ImmutableOffsetMap and SharedSingletonMap.

JIRA: YANGTOOLS-917
Change-Id: I8e7808b9bfe751885b842715f7e0346c0e7485b2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>