Refactor joiner to reduce symbol bloat.

[quickfur]

Using Steven Schveighoffer's "horcrux" trick. In my range-heavy project, this single change resulted in a 50% reduction in executable size(!). This is on top of the reductions already realized by my own application of the same trick (see forum discussion below).

Relevant discussions:

• Forum discussion 1
• Forum discussion 2
• Steven's blog post explaining the technique
• Relevant DMD pull (5855)

This PR is just one of many to come. Some of the most common range functions in Phobos are Voldemort types, which cause nasty symbol size bloat. These PRs aim to reduce this bloat and make heavily ranged-based code more viable for D. (At least until the DMD PR is pulled. But this problem is quickly becoming a showstopper for me in the meantime, because yesterday I started running into DMD's 10MB symbol size limit. Not to mention ELF errors caused by excessively large symbols.)

[dlang-bot]

Thanks for your pull request, @quickfur!

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[andralex]

Does @rainers recent work on compressed mangling help with this?

[PetarKirov]

This looks like a transformation that could be done automatically by dmd, not top of what dlang/dmd#5855 would do to improve the encoding of symbols, though for the time being we need workarounds such as this, so LGTM.

[wilzbach]

I agree with @ZombineDev here, the problem is real and this is a nice solution to solve this problem for the critical bits for the time being (I will comment more on the other PR with the longer discussion).

[rainers]

Does @rainers recent work on compressed mangling help with this?

Yes, the new mangling never repeats a type in the mangling.