Using string_view before actually requiring C++17 results in either a ton of additional ifdef (which mptString.h and mptString.cpp already have plenty enough of anyway), or another zoo of additional string types named mpt::ustring_maybe_view for each and every string type that we have and still then result in strange const string_view & parameters (or again another set of ifdef). I would rather delay that until we require C++17.

Using value parameters instead of const & for trivial iniline conversion functions pessimizes binary size for me: 4.447.744 bytes (before), 4.453.376 bytes (after) (VS2019 Win32 ReleaseShared).

Let's wait for C++17 then.