Reexport functions from Statistics

It does not make any sense to avoid exporting functions from Statistics
to which we add methods. These are documented in the manual but currently
not available without typing `using Statistics`.
The only method from Statistics which we do not overload is `middle`, which
this commit does not reexport for now.

src/StatsBase.jl

@@ -26,6 +26,20 @@ import StatsAPI: pairwise, pairwise!
 
 export
 
+    ## functions defined in Statistics
+    cor,
+    cov,
+    mean,
+    mean!,
+    median,
+    median!,
+    quantile,
+    quantile!,
+    std,
+    stdm,
+    var,
+    varm,
+
     ## weights
     AbstractWeights,    # abstract type to represent any weight vector
     Weights,            # to represent a generic weight vector

test/runtests.jl

@@ -2,7 +2,6 @@ using StatsBase
 using Dates
 using LinearAlgebra
 using Random
-using Statistics
 
 tests = ["ambiguous",
          "weights",