srf_index_hash adds and index to an SRF file or replaces an
existing index with a new one. In the case of concatenated SRF files
only the index at the end of a file will be replaced, but internal
indices will not be consulted by SRF tools.
The index is a hash table indexing the sequence names only. The name
itself does not appear in the index, rather the top 7-bits of a 64-bit
hash key are held in the index along with N-bits used to determine the
hash bucket. This reduces the index size to around 10-15 bytes per