Extract (some) Writer classes to separate files.

TODO: PNGWriter could probably reuse this stuff too, but...

firmware/application/capture_thread.hpp

@@ -26,19 +26,13 @@
 
 #include "event_m0.hpp"
 
-#include "file.hpp"
+#include "io.hpp"
 #include "optional.hpp"
 
 #include <cstdint>
 #include <cstddef>
 #include <utility>
 
-class Writer {
-public:
-	virtual File::Result<File::Size> write(const void* const buffer, const File::Size bytes) = 0;
-	virtual ~Writer() = default;
-};
-
 class CaptureThread {
 public:
 	CaptureThread(