Remove uses of std::string from exception throwing

[swuerl]

This uses the micropython-internal mp_obj_new_exception_msg_varg to format exception strings rather than std::string concatenation. Also changed the <class>_locals string to use micropython-native string concatenation.

[swuerl]

I'm again very confused about the CI, the missing reference exists here, also in the windows-pyd branch: https://github.com/stinos/micropython/blob/windows-pyd/py/vstr.c#L176

[stinos]

Dynamic linking with msvc or similar is different than for gcc: functions/data needs to be exported explicitly. Typical solution would be to add the missing function to micropython\ports\windows\msvc\exports.def which is annoying because it is in a separate repository.. Could be fixed with a pre-build event, but I've just added it to the windows-pyd branch so if built again this should now link fine.

[stinos]

Appveyor seems to have just updated their toolchain so this is now failing on micropython/micropython#16352..

[swuerl]

Ah makes sense. Thank you for looking into it, was just trying to figure out what's going on there.

I'll see if I can spin up a VM for windows testing; I'm currently relying on the CI here