Add aare::ssize_t alias for MSVC (#398)

- MSVC has no ssize_t, which made every translation unit fail on
Windows.
- Define aare::ssize_t in defs.hpp: the platform ssize_t on POSIX (so
nothing changes there) and std::ptrdiff_t on MSVC.
- Code inside the namespace picks it up unchanged; the few files that
use ssize_t outside the namespace get a using-declaration. FilePtr.hpp
now includes defs.hpp since it had no aare include.

Closing #393

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

python/src/bind_MultiThreadedFileReader.hpp

@@ -16,6 +16,7 @@
 #include <pybind11/stl/filesystem.h>
 
 namespace py = pybind11;
+using aare::ssize_t;
 
 inline py::dtype multi_threaded_reader_numpy_dtype(const aare::Dtype &dtype) {
     using aare::Dtype;