Improve performance of mstro_attribute_val_cmp_str
ECMWF's workflow shows excessive cost of
Avoid allocation for small attribute value types is the first attempt. We also need to check which comparisons trigger this exactly -- if would be great to have a self-contained reproducer that can be profiled outside of the ECMWF workflow.
avoid alloc/free for small types
check whether we could cache the value for repeated conversions on the same CDO/ same subscription
build reproducer for profiling?