Replace i128_support feature with compile-time detection

[vks]

This breaks builds using the i128_support feature with nightlies
supporting i128 before Rust 1.26. I think this is acceptable, unless we want to support very old nightlies.

[dhardy]

I argued against this before (#459) and I'm still not sure it's a good idea. The theoretical disadvantage is that projects may not support older compilers even though they trivially could.

In fact, this implementation is worse, in that it completely breaks builds with old nightlies using i128. This isn't necessary.

The status quo is that users needing i128_support must pass the feature flag, and this then works on current compilers, older compilers (only nightlies before 1.26), and in the future — at some point the flag will become redundant, and maybe eventually removed.

[vks]

The theoretical disadvantage is that projects may not support older compilers even though they trivially could.

As far as I can tell, this would only apply to nightlies older than 1.26.

In fact, this implementation is worse, in that it completely breaks builds with old nightlies using i128. This isn't necessary.

It's true that it's not necessary to break those nightlies, but do we really want to commit to supporting old nightlies? Note that in general this is not possible, because supporting new nightlies often means breaking old nightlies. Due to this, I think it is fair to assume that nightlies are at least as young as the latest stable release. (We should document this though.)

at some point the flag will become redundant, and maybe eventually removed.

It will only become redundant when we enable it by default, resulting in much worse breakage than this PR causes.

[dhardy]

I guess you're right; trying to support old nightlies is ridiculous. Maybe we should never even have allowed the i128_support feature flag on stable (in fact, with auto-detection, it needn't ever have been introduced).

Okay, then this has my approval. We could add a note in the README. We should in the CHANGELOG. I guess it makes sense to leave the dummy feature flag until later and remove in the 0.7 release.

Any further comments? Otherwise I'll merge in 1-2 days.

[vks]

Should we add "Nightlies older than the last stable version of Rust are not supported." to the README?

[dhardy]

But we can't even guarantee support for nightlies newer than the last stable version... so I don't think that's necessary.

Just a note in the changelog here then and we're done?

[vks]

I updated the changelog (for a few other changes as well) and rebased on master.