Add a method to get an immutable reference to the level 4 table

[pjht]

The current mappers provide a method to get a mutable reference to the level 4 table. I have found that a mutable reference is (at least for my usecases), unnecessary and causes borrow checker issues. I would like to add a method to get an immutable reference instead, maybe called level_4_table_immut.

[Freax13]

That seems reasonable. I think the most idiomatic way to deal with this would be to make level_4_table return an immutable reference and to add level_4_table_mut which returns a mutable reference, however that would be a breaking change.

We could either introduce that change for the next breaking release, add level_4_table_immut right now or do both, by adding level_4_table_immut now and later renaming level_4_table_immut and level_4_table to level_4_table and level_4_table_mut respectivly in the next breaking release.

[Freax13]

I went ahead and created #327. If merged this will be part of the next breaking release.