issue #106: make btree_container's size_type unsigned for consistency

parallel_hashmap/btree.h

@@ -837,7 +837,7 @@ namespace priv {
 
         using allocator_type = Alloc;
         using key_type = Key;
-        using size_type = std::make_signed<size_t>::type;
+        using size_type = std::size_t ;
         using difference_type = ptrdiff_t;
 
         // True if this is a multiset or multimap.
@@ -2755,7 +2755,7 @@ namespace priv {
             return {0, _begin};
         }
 
-        if (count == size_) {
+        if (count == (difference_type)size_) {
             clear();
             return {count, this->end()};
         }