`blob-s64-native-set!``blob``k``n`procedureK must be a valid index of blob; so must the indices {k, ..., k+ 7}. Endianness must be an endianness object.

These retrieve and set eight-octet representations of numbers at indices {k, ..., k+ 7}, according to the endianness specified by endianness. The procedures with u64 in their names deal with the unsigned representation, those with s64 with the two's complement representation.

The procedures with native in their names employ the native endianness, and only work at aligned indices:

`k`must be a multiple of 8. It is an error to use them at non-aligned indices.