Sereal::Merger: fix typo
Sereal::Merger: implement STRTABLE_purge

The idea is to clean up hash table after unseccessull append().

For example:
- we have N documents to merge.
- document M is broken (i.e. not a valid Sereal document for whatever reason)
- resulting document is R

We want to recover after appending M to R and failing doing so because M is invalid.

The main obstacle with recovery processes is that during merging M into R
we might have already added some records into string dedup hashtable (if
dedupe_strings => 1). Before next document can be merger, such records
have to be deleted from hash table. STRTABLE_purge allows doing so.

STRTABLE_purge uses several observations to optimize deletion:
- when merger fails to append M to R we want to delete all records
  whose offset is bigger than R's size before M was merged.
- offsets of STRTABLE_entries of an arena are continiosly growing

Using above mentioned information deletion became a simple task:
for (end_of_arenas ... start_of_arenas) {
    if (elem->offset < offset)
        break;

    delete elem
}