This saves memory, effort and locking.
Since every symbol is also added to postings, `Symbols()` can be
implemented there instead. This now has to build a map for
deduplication, but `Symbols()` is only called for compaction, and `gc()`
used to rebuild the symbols map after every compaction so not an
additional cost.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>