sol::var, sol::property, sol::as_function, improvements

This release adds some minor touchups to performance, removes the need for RTTI (we can get type names without it now), exposes a function-disambiguator (useful for lambdas) called as_function and adds a new feature, sol::var.

sol::as_function was inspired by @Nava2 needing to handle functions better, and it turned out to help the next person mentioned for the other feature as well! Thanks for bringing it up.

sol::var was inspired by https://twitter.com/eevee and is something I probably should have caught sooner, but yay twitter (thanks @eliasdaler for looping me into the conversation)!

Also noteworthy is that @eevee also pointed out that lambdas should work with property, not just member variables/functions. So, that's now fixed and ready to go too!

Everyone should upgrade, since this is a non-breaking usability improvement.