Implement Sodium::Buffer[]= on top of Sodium::FFI::Memory#sodium_memput
Implement Sodium::Buffer[]= on top of Sodium::FFI::Memory#sodium_memput

Sodium::Buffer[]= will be incredibly useful to have for performant
buffer manipulation (by avoiding having to repeatedly allocate and
concatenate strings). It has been implemented on the new memput function
exposed through C.

stouset/sodium,stouset/sodium