Fixed some more MSVC warnings (#24)

Author: improbable-til

phtree/phtree_test.cc

@@ -62,7 +62,7 @@ struct Id {
         ++default_construct_count_;
     }
 
-    explicit Id(const size_t i) : _i{(int)i} {
+    explicit Id(const size_t i) : _i{static_cast<int>(i)} {
         ++construct_count_;
     };
 

phtree/phtree_test_unique_ptr_values.cc

@@ -42,7 +42,7 @@ class IntRng {
 struct IdObj {
     IdObj() = default;
 
-    explicit IdObj(const int i) : _i(i){};
+    explicit IdObj(const size_t i) : _i(static_cast<int>(i)){};
 
     bool operator==(IdObj& rhs) {
         return _i == rhs._i;