Port a subset of basic std::fs functionality (theseus-os#510)

* Add a Theseus-specific port of select `std::fs` functionality, including basic file I/O (read, write, seek) 
* Add our own fork of `core2` with expanded `std::io` traits

.gitmodules

@@ -25,3 +25,6 @@
 [submodule "libs/thiserror-core2"]
 	path = libs/thiserror-core2
 	url = https://github.com/theseus-os/thiserror-core2.git
+[submodule "libs/core2"]
+	path = libs/core2
+	url = https://github.com/theseus-os/core2.git

Cargo.toml

@@ -6,6 +6,7 @@
 members = [
 	"kernel/[!.]*/",
 	"applications/[!.]*/",
+	"ports/theseus_std",
 ]
 
 
@@ -59,6 +60,8 @@ smoltcp = { git = "https://github.com/m-labs/smoltcp" }
 
 ### Patch `libc` so we can use libc-specific types when using `cfg(target_os = "theseus")`.
 libc = { path = "ports/libc" }
+### Patch `core2` with newer functions from `std::io`, e.g., additional `Seek` trait functions
+core2 = { path = "libs/core2" }
 
 ##############################################################################################
 #################### Below are patches for wasmtime-related crates. ##########################

kernel/io/src/lib.rs

@@ -49,7 +49,7 @@ extern crate lockable;
 #[cfg(test)]
 mod test;
 
-use core::{borrow::Borrow, cmp::min, marker::PhantomData, ops::{Deref, Range}};
+use core::{borrow::Borrow, cmp::min, marker::PhantomData, ops::{Deref, DerefMut, Range}};
 use alloc::{boxed::Box, string::String, vec::Vec};
 use lockable::Lockable;
 use core2::io::{Seek, SeekFrom};
@@ -478,9 +478,11 @@ impl<RW> BlockWriter for ByteWriterWrapper<RW> where RW: BlockReader + BlockWrit
 /// A readable and writable "stateful" I/O stream that keeps track 
 /// of its current offset within its internal stateless I/O stream.
 ///
-/// This implements the [`core2::io::Read`] and [`core2::io::Write`] traits for read and write access,
-/// as well as the [`core2::io::Seek`] trait if the underlying I/O stream implements [`KnownLength`].
-/// It also forwards all other I/O-related traits implemented by the underlying I/O stream.
+/// ## Trait implementations
+/// * This implements the [`core2::io::Read`] and [`core2::io::Write`] traits for read and write access.
+/// * This implements the [`core2::io::Seek`] trait if the underlying I/O stream implements [`KnownLength`].
+/// * This also forwards all other I/O-related traits implemented by the underlying I/O stream.
+/// * This derefs into the inner `IO` type, via both [`Deref`] and [`DerefMut`].
 pub struct ReaderWriter<IO> {
     io: IO,
     offset: u64,
@@ -491,6 +493,17 @@ impl<IO> ReaderWriter<IO> where IO: ByteReader + ByteWriter {
         ReaderWriter { io, offset: 0 }
     }
 }
+impl<IO> Deref for ReaderWriter<IO> {
+    type Target = IO;
+    fn deref(&self) -> &Self::Target {
+        &self.io
+    }
+}
+impl<IO> DerefMut for ReaderWriter<IO> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.io
+    }
+}
 impl<IO> core2::io::Read for ReaderWriter<IO> where IO: ByteReader {
     fn read(&mut self, buf: &mut [u8]) -> core2::io::Result<usize> {
         let bytes_read = self.io.read_at(buf, self.offset as usize)