Add object-safe traits for File, Filesystem, Storage

The Storage trait is not object safe, and the File and Filesystem struct
have the Storage implementation as a type parameter.  This can make it
hard to write code that works with multiple storage implementations, and
it can also lead to increased binary size as the filesystem operations
are monomorphized for all storage implementations.

This patch introduces three new traits, DynFile, DynFilesystem and
DynStorage, that are object-safe wrappers for the File, Filesystem and
Storage types.  Some functionality cannot be expressed directly in these
traits and is moved into helper functions instead, e. g. callbacks with
arbitrary return types or methods using const generics.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+## Added
+- Added object-safe traits `DynFile`, `DynFilesystem` and `DynStorage` for
+  accessing `Storage`, `Filesystem` and `File` implementations for any storage.
+
 ## Fixed
 
 - Fixed macro hygiene for `path!`.

src/lib.rs

@@ -141,9 +141,9 @@ mod c_stubs;
 
 pub mod consts;
 pub mod driver;
-
 pub mod fs;
 pub mod io;
+pub mod object_safe;
 pub mod path;
 
 /// get information about the C backend