add pgx.CopyFromSlice

Using CopyFromRows can often be inconvenient to use, because you would
need to convert a typed array to an [][]interface{}. Similarly,
implementing a custom CopyFromSource is too verbose for one-off things.

Add CopyFromSlice that allows to more easily convert a slice to a
CopyFromSource. Example:

    copyCount, err := conn.CopyFrom(
        context.Background(),
        pgx.Identifier{"people"},
        []string{"first_name", "last_name", "age"},
        pgx.CopyFromSlice(len(rows), func(i int) ([]interface{}, error) {
            return []interface{user.FirstName, user.LastName, user.Age}, nil
        }),
    )

doc.go

@@ -260,6 +260,22 @@ interface. Or implement CopyFromSource to avoid buffering the entire data set in
         pgx.CopyFromRows(rows),
     )
 
+When you already have a typed array using CopyFromSlice can be more convenient.
+
+    rows := []User{
+        {"John", "Smith", 36},
+        {"Jane", "Doe", 29},
+    }
+
+    copyCount, err := conn.CopyFrom(
+        context.Background(),
+        pgx.Identifier{"people"},
+        []string{"first_name", "last_name", "age"},
+        pgx.CopyFromSlice(len(rows), func(i int) ([]interface{}, error) {
+            return []interface{user.FirstName, user.LastName, user.Age}, nil
+        }),
+    )
+
 CopyFrom can be faster than an insert with as few as 5 rows.
 
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