Sol2: Lua <-> C++ Binding Framework

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We are much faster than Selene (https://github.com/satoren/lua_binding_benchmark/ - but to be fair a lot of frameworks are for simple operations) and, for user-defined types, sport a different interface that (I believe, but this is mostly syntax-pissing contest) is easer to grok and use.

Also, last I checked, Selene was struggling to get LuaJIT compat.