Update arrayvec to 0.5.1

[NikVolf]

This will also cancel the dependency of nodrop, which is deprecated there

[cesarb]

Unfortunately, it also increases the minimum Rust release to 1.36; currently, this crate works on at least 1.20 (tested by travis-ci), and might even work on earlier releases. So this change can only be merged on the next incompatible release. Since the latest 0.4.x versions of arrayvec seem to have all the relevant fixes (through a build.rs which autodetects Rust 1.36 or later), and the only user of arrayvec is not part of this crate's API, I don't see any urgency.

[sylvestre]

@cesarb relying on very old crates make packaging for linux distro harder. arrayvec is used by other crates with the more recent version, blake2-rfc requires an old version which isn't compatible with the new version.

It forces us to either patch a crate, or to package two versions of arrayvec