New creates a new `ExtSorter`.

NewMultiWayMerge initialize a new `MultiWayMerge` instance.

Spawn spawns a new goroutine to do the external sorting, returns two channels for user to interact.

DeltaDecoder decodes delta-encoded keys.

DeltaEncoder applies delta encoding to a sequence of keys.

ExtSorter implements external sorting.

MultiWayMerge implements k-way merge using min-heap provided by golang builtin library, it implements `heap.Interface` interface.

Options defines configurable options for `ExtSorter`.