Auto merge of #962 - christianpoveda:file-shim, r=oli-obk

Add shims for file handling

This adds the bare minimum to be able to do `File::open` and `File::read`. I also need some feedback about how to handle certain things

tests/hello.txt

@@ -0,0 +1 @@
+Hello, World!

tests/run-pass/file_read.rs

@@ -0,0 +1,13 @@
+// ignore-windows: File handling is not implemented yet
+// compile-flags: -Zmiri-disable-isolation
+
+use std::fs::File;
+use std::io::Read;
+
+fn main() {
+    // FIXME: create the file and delete it when `rm` is implemented.
+    let mut file = File::open("./tests/hello.txt").unwrap();
+    let mut contents = String::new();
+    file.read_to_string(&mut contents).unwrap();
+    assert_eq!("Hello, World!\n", contents);
+}