Reading files!

- File.close()
- File.open()
- File.read()
- file.readBytes()

And a few other little methods. Still lots more work to do, but it's a
start.

test/io/file/read.wren

@@ -0,0 +1,8 @@
+import "io" for File
+
+var text = File.read("test/io/file/file.txt")
+System.print(text) // expect: this is a text file
+System.print(text.count) // expect: 19
+
+// TODO: A file containing line endings.
+// TODO: A file containing null bytes.