rlua::Lua needs compile time option for Send or not Send

[jugglerchris]

As copied from #172 (comment)

rlua::Lua needs a compile-time switch to either require Send or to not require Send. Not-requiring Send seems like the obvious choice, but it makes it very hard to integrate into game engines that use threaded systems or perhaps into things like web servers.

[khvzak]

rlua is going to be deprecated soon in favour of mlua, where this feature is already supported (via send feature flag).