Verify all methods are checking stack as appropriate
Add simple example that runs Lua code input on the command-line
Implement register/getglobal/setglobal
Change how tostring() works

Make tostring() always return a &str. Clients can map it with
.to_owned() if they want a ~str.

Add method tobytes() that returns a &[u8], for dealing with Lua strings
that don't contain utf-8.

Add method .pushbytes() that pushes a &[u8] as a lua string.
Fix early free in loadfile_unchecked

The CString was being dropped too early, which is not good at all.
Remember to use .as_ref() before mapping the option to prevent it from
being consumed.
Add a bunch of the check* methods