Missing Floor / Ceil methods

[Razaekel]

There's no Ceil or Floor method implemented for the wide types.

Based off of Issue #15, it appears that only Intel has intrinsics for floor/ceil, and other architectures will need a manually implemented version.

[chrisranderson]

@Lokathor Would this be a "good first issue"? Seemed like a decent candidate to me. I'm looking to contribute to a Rust project during my free time. Just wanted to check before diving in too far.

[Lokathor]

I'd be happy to have the additional methods!

Let me also point out that, long term, a crate similar to this one will become part of the core standard library. https://github.com/rust-lang/project-portable-simd is where the other effort is going on, which might let your contributions have a wider impact.