Allow passing a StatBlock to LuaEngine::exec()

To pass the StatBlock data to the lua script itself, we create a lua
table and add select variables from the StatBlock to it. Right now, only
the most common stats are added (e.g. hp/mp, phys/ment/off/def, etc).
Future StatBlock refactoring may let us add some of the newer stats.

This commit also changes things so that we don't try and reuse the lua
state. Instead, we create a new state when calling exec() and close it
when returning the result.