Removing mut fields from vec.rs, at_vec.rs, str.rs, unstable.rs, and cell.rs.
Unstable didn't need transmute_mut after all.
auto merge of #5698 : Thiez/rust/de-mutfielding, r=pcwalton

This pull request removes some mut-fields from at_vec, str, vec, unstable, and cell. Sadly in case of Cell this required using either transmute_mut (2 instances) or changing the interface. I chose the former. Perhaps it would be a good idea to merge Cell and Option, and take that opportunity to change the interface to use '&mut self' instead of '&self' (which would enable removing the transmutations) for take and put_back.

r?