The fst crate recently added support for sliced `Arc<Vec<u8>>`.
This called for a rewrite of the RAMDirectory for tantivy's RAMDirectory.
Previously every single read was copying data.
In addition:
- RAMDirectory's Write object panic if someone does not flush
right before the destruction of the object.
- In the same spirit, the postings serializer panics if someone
opens a term without closing the previous one.
Closes#16