On Windows, encode file paths as UTF-16 before trying to open the file
for reading/writing.
This introduces a new class `blender::fstream`, which wraps
`std::fstream` and provides this UTF-16 encoding. This class should also
be used in other areas, like the Alembic importer/exporter.
Manifest Task: T93960
Reviewed By: JacquesLucke
Differential Revision: https://developer.blender.org/D13633
