Rename the following:
LuaNil => Nil
LuaExternalError => ExternalError
LuaExternalResult => ExternalResult
LuaCallback => Callback (internal only)
Use qualified re-exports at the top of the module.
Add a new public 'prelude' module which re-exports everything with a
non-conflicting name (Adds back the Lua prefix), and is meant to be imported
unqualified.
For this to work, both `T` and `E` need to implement `ToLua`. An `Ok(t)`
passes the contained `T`, while an `Err(e)` passes `nil` followed by the
contained `E`. This matches the common Lua idiom used by functions like
`io.open`.
Closes#3
kyren