Remove custom Lua wrapper and use sol2 exclusively

[cschreib]

Several drawbacks of our current Lua wrapper:

• The tools for writing glue functions (lua::function) are not super well designed and will likely have a larger runtime overhead compared to sol2.
• It is not easy to write a Lua glue function.
• It relies on Lunar for exposing C++ classes to Lua. Lunar is not maintained.
• Lunar has no concept of inheritance. Inherited glues must be explicitly listed in derived classes.
• The way we use Lunar, we need a wrapper Lua glue class for each C++ class, this is not easy to maintain.

Advantages of our wrapper over sol2:

• Error messages on incorrect parameters are likely to be nicer (showing list of possible types, etc.).