added os.open(); new file object

eg.
```lua
local err, file = os.open("LICENSE.md")
if err then
    // do error handling stuff
end

// passing "a" to read() will read the whole file
print(file:read("a"))
```

examples/reader.cosmo

@@ -0,0 +1,9 @@
+
+local err, file = os.open("LICENSE.md")
+print("made file")
+if err then
+  print("failed to open file")
+end
+
+print(file)
+print(file:read("a"))