Remove mutability restrictions for some methods of state

[therustmonk]

Fixes #46

I've updated:

• All of is_ methods, because it never change the state
• Some to_ methods, which returns integers, bools or c-function pointer

I didn't change group of check_ and opt_, because it can raise error which will mutate state.

[SpaceManiac]

I think all the to_ methods actually mutate the value at the specified index into that type if possible, and should not be included in this PR.

Further, while the rest of the methods don't mutate anything as far as I can tell, I'm not really sure of the usefulness of this change. Are there any usecases that are enabled, or is it just for documentational clarity?

[therustmonk]

I reverted mut to to_ methods. It looks inconsistent if some needs mutable, but others not. Most of to_ methods (maybe all) really can change stack or raise error.

It's for documentation clarity at first, also to make type checking closures less verbose and to make possible parallel checking.

[jcmoyer]

I am not sure what the best practices for this are. It doesn't seem like we can make guarantees about the behavior of Lua's functions between releases. Changing back to mut from non-mut is a breaking change.

[therustmonk]

Umph... sounds resonable! That's ok if you reject it.

[therustmonk]

I've closed it, because we have no any garantee about mutability inside Lua.