cleanup clang-y static analysis

Some performance improvements.  eg. pass by ref.

Some paranoia pre-initialization of variables
which will be initialized later by indirection.

And some misc. cleanup.

src/datafmt.cpp

@@ -110,7 +110,7 @@ struct FmtDelta {
         field(prefix, val, verytop);
 
         if(val.type()==TypeCode::Struct) {
-            for(auto fld : val.imarked()) {
+            for(const auto& fld : val.imarked()) {
                 std::string cprefix(prefix);
                 if(!verytop)
                     cprefix += '.';