Use the stable stdx-allocator DUB package

[wilzbach]

As discussed before, stdx-allocator is a frozen version of std.experimental.allocator to avoid the frequent breakages.

See also: dlang-community/discussions#27

[wilzbach]

Hmm this doesn't seem to work with DMD 2.071 (and thus LDC 1.1):

While the non-existent Ternary problem and a couple of other issues with an older Phobos can be fixed easily, one then still runs into safety issues with 2.071.2:

I don't really fancy wasting time looking into these and would only do so if dropping support for 2.071.1 isn't an option. I just pushed dlang-community/stdx-allocator@d06e4f2, which allows building stdx-allocator with 2.072.2.

Let me know how you would like to proceed.

[s-ludwig]

I've opened #45 to drop 2.071 support.

[wilzbach]

I've opened #45 to drop 2.071 support.

Thanks! (Rebased this PR)

[wilzbach]

Everything passing 🎉

[wilzbach]

Tagged this as v1.4.0-alpha.1, s.t. it can be used by Vibe.d