[WIP] Add a LuaMutex type to borrow the InsideCallback

[tomaka]

Fix #10

Blocked by https://github.com/rust-lang/rust/issues/38673

Changes are:

• The parameters of a Rust function passed to Lua must now implement LuaRead<&InsideCallback> (right now it's LuaRead<&mut InsideCallback> + 'static, which is a hack)
• All basic types (String, integers, etc.) already implement LuaRead<&'a InsideCallback> so they will still work.
• Types such as LuaFunction or LuaTable do not implement LuaRead<&'a InsideCallback>.
• A new type LuaMutex<T> (where T can be LuaFunction or LuaTable) does implement LuaRead<&'a InsideCallback>.
• The LuaMutex struct has a lock function that locks the InsideCallback and reads the T. As long as the T is alive, no other LuaMutex can be locked.

So for example if you want a Rust function that takes two tables as parameter, you can pass a closure like |(table1: LuaMutex<LuaTable<_>>, table2: LuaMutex<LuaTable<_>>)|. You can then read for example table1 by calling table1.lock(). The lock function will return a LuaTable.

As long as the LuaTable is alive, table2 can't be locked.

See #10 for an explanation about why this is necessary.

A future change is to allow LuaRead<&mut InsideCallback> if there is only one parameter, which would allow closures with only one parameter to bypass this LuaMutex system.

[tomaka]

Also added the 'lua lifetime to InsideCallback to fix soundness.

[tomaka]

I managed to make it work for InsideCallback<'static> only. Since this is the most common case, I'm going to push the change and fix this later.

[tomaka]

The current code is still unsound because it allows reading and writing the same userdata simultaneously.