Improve performance of mstro_attribute_val_cmp_str

ECMWF's workflow shows excessive cost of mstro_attribute_val_cmp_str.

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?