Remove dependency on marshal and push the Lua error handler only once

[ShadowNinja]

This makes a number of changes:

• Remove the dependency on marshal by using string.dump and loadstring.
• Use lua_tolstring rather than having Lua functions pass string lengths to C++.
• Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
• Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
• Only push the error handler once per Lua stack by storing it's index in ScriptApiBase::m_errorhandler.
• Code style fixes
• Pop some more items from the stack
• Other misc changes

I am unsure of whether the check for broken LuaJIT versions is still relevant, as string.dump may not be broken on those versions.

[ShadowNinja]

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