hecl/hecl: Slightly improve resource usage within string conv operator+ funcs
hecl/hecl: Slightly improve resource usage within string conv operator+ funcs

We already construct a std::string instance, so we can just append to
it instead of creating another temporary with std::string's operator+.
We also change this to append using the string view getter functions, as
this allows the appending process to do less work. When a pointer is
passed in, a strlen call would need to be performed in order to
determine the total characters to append. However, we already know the
size (via the string view).

AxioDL/PathShagged,AxioDL/PathShagged,AxioDL/PathShagged