Add context and file structs

kernel/context/file.rs

@@ -0,0 +1,10 @@
+//! File struct
+
+/// A file
+#[derive(Copy, Clone, Debug)]
+pub struct File {
+    /// The scheme that this file refers to
+    pub scheme: usize,
+    /// The number the scheme uses to refer to this file
+    pub number: usize,
+}