Instead of storing pg_statistic stavalues entries as text strings, store

Instead of storing pg_statistic stavalues entries as text strings, store
them as arrays of the internal datatype.  This requires treating the
stavalues columns as 'anyarray' rather than 'text[]', which is not 100%
kosher but seems to work fine for the purposes we need for pg_statistic.
Perhaps in the future 'anyarray' will be allowed more generally.