ctfe append too slow, but can't speed up

[Jethro via Digitalmars-d-learn]

ctfe string appending is way to slow, I have tried the suggested 
methods and nothing works and slows down by at least an order of 
magnitude.

I need a drop in replacement(no other changes) that can take over 
the duties of string and allow for memory reuse.

reserve, cpacity, assumeSafeAppend cannot be used by ctfe so are 
useless.

I've tried to shadow a pre-allocated buffer but that doens't work 
either.