Implement minimal patch to get `tz.cpp` to work with Windows + Unicode

[DavisVaughan]

Closes #10

This is a minimal ish patch to get date to work with wide character file paths on Windows when one is supplied as the install destination through set_install(const std::string& path).

It mainly required two things:

• A to_utf16() converter to convert from std::string to std::wstring

• A subclass of std::streambuf that can stream from a FILE*, that we can then use in std::istream. This is explained more in the code, but essentially std::ifstream's constructor does not allow wide character file paths to stream from, so we have to create our own file streambuf class that can open wide character file paths, and then use that in std::istream.

I don't understand all the details of the streambuf class, but it seems to be working?

Once we have those two helpers, we just use them any time we have to access the path provided by set_install().

[gaborcsardi]

Btw. I think it would make sense to test this on Solaris, and also on a non-UTF-8 Linux, e.g. R-hub has an image, before the release.

[DavisVaughan]

Passing with the test on both Solaris and debian-clang-devel https://builder.r-hub.io/status/tzdb_0.1.2.9000.tar.gz-c681d4eaa6404f87a8b9cadb5406b0d3 https://builder.r-hub.io/status/tzdb_0.1.2.9000.tar.gz-599121d923d842f6b73e346ecb967eeb