Answer a value MEOS writes through a pointer to a pointer

[estebanzimanyi]

Some accessors answer a value that is itself a MEOS value: set_value_n on a
set of geometries writes a GSERIALIZED **, ttext_value_at_timestamptz a
text **, and their kin the same for a circular buffer, a network point, a
pose and a jsonb. The buffer holds the ADDRESS of the value rather than the
value, so the reader takes that address out of it and wraps it in the class the
model gives the type — the same wrap a return of that type gets. 19 accessors
reach the object layer that way, so GeomSet.ValueN answers a Geo and
TText.ValueAtTimestamptz a Text, each null where MEOS says the value does
not exist. 1190 methods against 1171, 105 deferred against 124.

Two tests read both answers: a geometry set answers the geometry at an index
and null past the end, and a temporal text answers its value inside its span
and null outside.