Interacting with Lua using its low-level C API quickly becomes cumbersome
and error-prone. In such cases introducing an object-oriented glue layer
makes accessing Lua state considerably easier.

This commit adds several helper classes to campvis-scripting. The abstract
LuaTable and its concrete sub-classes RegularLuaTable and GlobalLuaTable
should make working with Lua tables more pleasant by reducing common
operations to a single method call. LuaVmState's task is to simplify the
management of Lua VM's state with various high level methods that do
automatic error checking.

The new classes are largely undocumented. This will be fixed in a future
commit.

References #1