Indexed-gzip

* New `IndexedGzipFile.seek_points` method, which returns the compressed and
uncompressed seek point locations.

* `IndexedGzipFile` objects are now picklable, as long as they are created with
the default setting of `drop_handles=True` and with a `filename` and not an
open `fileobj` (28, 31).
* Changed the return type of `zran_tell` from `long` to `uint64_t`, because the
former is not guaranteed to be 64 bit (29, 30).
* Changed the `zran_index_t.compressed_size` and `uncompressed_size` fields from
`size_t` to `uint64_t` because the former is not guaranteed to be 64 bit.

* Removed the deprecated `fid` argument to the `IndexedGzipFile` constructor.
* Removed the `SafeIndexedGzipFile`.

* Fix to package metadata.

* The `IndexedGzipFile.import_index` method and `zran_import_index` function
can handle index files which do not contain any index points (18).

* Fixed bug affecting files which were an exact multiple of the read buffer
size (15).