Replace `ring::polyfill::fill_from_slice` with use of `<[T]>::clone_from_slice()`

[briansmith]

See http://blog.rust-lang.org/2016/03/02/Rust-1.7.html. <[T]>::clone_from_slice() was stabilized in Rust 1.7, which is now the minimum supported version in ring.

Note that clone_from_slice may not actually get optimized as well as fill_from_slice does. If that is found to be the case, then we'll have to wait for rust-lang/rust#31755 to be resolved.

[briansmith]

Thanks! Landed in 7cdaeeb.

[Ms2ger]

Actually landed in 43af354.

[briansmith]

I am going to revert this. Although it didn't seem to affect crypto_bench numbers, it did negatively affect the amount of time Travis CI debug builds take and now some of them are timing out:
https://travis-ci.org/briansmith/ring/builds/114456084.

Looks like we'll have to wait for copy_from_slice to become stable. The ring issue for switching to copy_from_slice is #141.

Thanks for the patch though! Sorry I didn't notice this before.