ObjImporter: properly implement string-to-number on Android.
ObjImporter: properly implement string-to-number on Android.

The string stream solution that was used in case of NaCl and Android
wasn't working at all, thus I discarded it in favor of less overhead-y C
functions. While strtoul() and others *are* defined in the std::
namespace, strtof() isn't. What a pile of crap. I also had to emulate
C++ exception behavior in order to match the std::stoul() and
std::stof() behavior.

This plugin needs a zero-copy rewrite anyway, but at least the test is
green now on all platforms.

I discarded the NaCl version because currently there's no toolchain for
it and I'm not going to guess whether stof() and friends are even there.
If they are, then it's just a matter of enabling the Android code path
also for NaCl.

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