Multiple Lua versions

[SolraBizna]

This is related to #2. Aleph Bet's current Lua scripting implementation uses only (modified) Lua 5.2. This misses out on some features and improvements that were added in later versions. Upgrading to a later Lua version is not possible without breaking at least some older scripts.

With a lot of care and grunt work, it would be possible to decouple the Lua version from the Lua scripting interface. We could then have individual scripts choose whether they are run with Lua 5.2 or (say) Lua 5.4. LuaJIT could even be an option (with the caveat that it is not available on all architectures, and doesn't include post-5.1 features).

Primarily for namespacing reasons, the requisite work would be easier if it were done in Rust (#10 strikes again).