feat(dyngen): preconvert path to OsStr when opening the library

In libloading 0.9 the argument bound changed from `AsRef<OsStr>` to
`libloading::AsFilename`. This was necessary in order to enable
compilation of libloading for usecases that cannot depend on libstd.
`libloading::AsFilename` supports most of the same types, but cannot be
implemented generally for any `T: AsRef<OsStr>` due to the orphan rule.

This makes the generated code support both 0.8 and 0.9 versions of
libloading transparently, and at the same time it also reduces the
number of different instantiations of `Library::new`.

When bindgen is adjusted to work with libloading 0.9 only it will be
worthwhile to adjust the generation of `from_library` to use `c"string"`
literals over `b"string\0"` ones as those can forgo any runtime checking
of the argument(s). However we can't do it right while libloading 0.8 is
still supported.

Author: nagisa