v0.2.0

This change overhauls the API based on feedback of the original v0.1.0 release.

Breaking changes

• The public constructor for the KWayMerger is now the new kway_merge function.
  KWayMerger is public, but unexported.
• Instead of the F parameter (and argument to its constructor), kway_merge
  uses the same ordering API as Base's sorting functions.
• KWayMerger{T} now iterates @NamedTuple{from_iter::Int, value::T}, to reduce
  the risk of users conflating the two elements of the tuple.