Allow disabling of asm, core_intrinsics and const_fn to build on stable

[Ryman]

This also disabled Once when const_fn is not active as it doesn't seem to be useful without it (and there's no tests exercising it without static usage).

This shouldn't break anything as the default features are opt-in and should give the same behaviour as before.

[Ericson2314]

Nice! How about https://github.com/reem/rust-unreachable for unreachable!()? It works equally well on nightly or stable.

[Ericson2314]

Or even better, https://github.com/reem/rust-debug-unreachable

[Ryman]

I considered the first one but didn't want to add a dependency, but I'm happy to change to that if you want. The second one looks good, but it's not no_std... yet!

[Ericson2314]

Well, the final call is up to @mvdnes, but I never worry about the number of dependencies---traditionally, everyone using C rolled their own spinlock for just that reason!. Plus, with rust-lang/rfcs#1216 on the horizon, I expect that the first or both of those crates will be subsumed by core.

[mvdnes]

Nice work!

On the unreachable side:
The unreachable! macro seems fine, but I understand some performance reasons.
We could also copy the two relevant functions from the dependencies. Something like this:

#[inline]
unsafe fn unreachable() -> ! {
    enum Void { };
    let x: &Void = std::mem::transmute(1usize);
    match *x {}
}

[mvdnes]

I have merged this without modifications to the unreachable part. If someone has a good idea to update it, it can always be added later.

Anyway, thanks! 👍