fs/Path: rename to AllocatedPath

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

src/DecoderThread.cxx

@@ -27,7 +27,7 @@
 #include "system/FatalError.hxx"
 #include "Mapper.hxx"
 #include "fs/Traits.hxx"
-#include "fs/Path.hxx"
+#include "fs/AllocatedPath.hxx"
 #include "DecoderAPI.hxx"
 #include "tag/Tag.hxx"
 #include "InputStream.hxx"