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"))
```

src/cbaselib.h

@@ -3,6 +3,13 @@
 
 #include "cstate.h"
 
+enum
+{
+    COSMO_USER_NONE, // CObjObject is not a userdata object
+    COSMO_USER_FILE, // CObjObject is a file object (see cosmoB_osOpen)
+    COSMO_USER_START // the first user type for user-defined userdata
+};
+
 /* loads all of the base library, including:
     - base library ("print", "assert", "type", "pcall", "loadstring", etc.)
     - object library