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_bytes.wren

@@ -0,0 +1,13 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+
+System.print(file.readBytes(3)) // expect: thi
+
+// Always reads from the beginning.
+System.print(file.readBytes(7)) // expect: this is
+
+// Allows zero.
+System.print(file.readBytes(0).bytes.count) // expect: 0
+
+file.close()