[RFC] Improve compatibility with legacy versions

[clue]

Because why not… :-)

Now more seriously: This project is a low level lib that is used as a building block for quite a few higher level abstractions on top of it. As such, compatibility (even with significantly outdated versions) is a major concern to me.

Note that I'm not suggesting putting significant amount of work into this. The patch is already here and, personally, I see little harm in supporting this.

Also note that I'm not suggesting we need to keep support indefinitely. Should this ever turn out to be a burden in the future, e.g. because we actually require any new language features or some external lib, then I'm all for dropping support again.

[kelunik]

Note that it installs react/promise@v1.2.1, as the current version requires php>=5.4. Also, will requiring PHP 5.4 then be considered as a BC break and require a new major version?

[WyriHaximus]

@clue While the changes are small I do like to move forward somewhere this year, preferable to PHP 7+.

[WyriHaximus]

@clue could you merge in master and resolve the merge conflicts?

[clue]

Rebased now that #100 is in

@kelunik Yes, raising the minimum required version would technically be a BC break again. But as @WyriHaximus has pointed out, we will likely bump to PHP 7/7.1 at some (future) point anyway 👍