v0.18.15

DataStructures v0.18.15

Diff since v0.18.14

Closed issues:

• remove the <:Number constraint in Accumulator (#859)

Merged pull requests:

• Permit non-number types in Accumulator (#860) (@Krastanov)
• do not drop repeat keys in Accumulate(::Pairs...) (#861) (@Krastanov)
• remove eltype definition from SortedDict and SortedSet (#863) (@andyferris)