`boxfnonce` obsolete with release of Rust 1.35.0

[Sachin-Bhat]

Hi,

I noticed that version 0.11 of this crate started using the boxfnonce crate and this crate is obsolete.
(https://rustsec.org/advisories/RUSTSEC-2019-0040). Please consider using Box<dyn FnOnce(...) -> ...> in the relevant places instead.