Investigate and fix address of temporary warnings

Clang gives two legit warnings about using addresses of temporaries:

1) CtxSetting::get() returns `{ NullTimingCtx, settings::node<T>(T{}) }` when nothing is found, the second parameter is stored as const reference.

2) In IoSerialiserYAS.cpp, deserialise() contains

```
.fieldName         = fmt::format("{}[{}]", field.fieldName, i),
```

Here fieldName is a `std::string_view`.