deprecate and test top

Author: oxinabox

src/DataStructures.jl

@@ -112,5 +112,4 @@ module DataStructures
     include("splay_tree.jl")
 
     include("deprecations.jl")
-@deprecate top(x) = first(h)
-end
+end

src/deprecations.jl

@@ -1,3 +1,4 @@
 # 0.18 deprecations. Remove before releasing 0.19
 @deprecate path(t::Trie, str::AbstractString) partial_path(t::Trie, str::AbstractString)
 @deprecate find_root find_root!
+@deprecate top first

test/test_deprecations.jl

@@ -14,4 +14,9 @@
     @test collect(path(t, "robb")) == [t0, t1, t2, t3]
     @test collect(path(t, "ro")) == [t0, t1, t2]
     @test collect(path(t, "roa")) == [t0, t1, t2]
+end
+
+@testset "top" begin
+    hh = BinaryMinHeap{Float64}([1,2,3])
+    @test top(hh) == 1
 end