Make `entries` binding immutable for &mut type
Make `entries` binding immutable for &mut type

It appears that the lint on nightly is more aggressive and it correctly
identifies that the entries binding itself is not mutated even though
the type is (and needs to be) &mut.

steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io