Investigate and fix address of temporary warnings

[frankosterfeld]

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.