Added support for use with no_std

[regexident]

This PR does three things:

1. It makes fn replace_with() & fn replace_with_or_default() available on both stable and nightly Rust with no_std.
2. It makes fn replace_with_or_abort() available on nightly Rust with no_std via "nightly" feature.
3. It makes unsafe fn replace_with_or_abort_unchecked() available via "panic_abort" feature.

The latter expects panic = "abort" to be defined in Cargo.toml. Being hidden behind a feature flag it will also be hidden on docs.rs (afaik), which I would consider desirable, actually. It's a "last resort" escape hatch, not an API one would want to get into everybody's hands, without knowing the issues around unwinding.

I chose the unsafe fn …_unchecked() naming pattern, as it's a quite common pattern already.

[alecmocatta]

Thanks for the PR @regexident!

I appreciate your explanation and hiding from the docs of replace_with_or_abort_unchecked. I wouldn't be surprised if replace_with_or_abort optimised equivalently under panic = "abort" in release builds. Have you observed this not to be the case? Or does something else about your use case still require it?

Regardless, the PR looks well thought through so I'm very happy to go ahead and merge and release shortly.

[alecmocatta]

Merged and released.

After my comment I realised you indended replace_with_or_abort_unchecked for stable no_std rather than for performance. That's fine by me. I am looking forward to deprecating and removing it once aborting/trapping is available in stable no_std. Linking relevant issue rust-lang/rfcs#2512 for future reference.