Bug 18342 – std.concurrency needs to be optimized

I recently used D on a C++ performance Meetup where they were testing submitting functions to a worker thread and guess what - D was 5x slower than C++. There's lots of slow stuff going on in `std.concurrency` - it uses double mutex-locking - once in `put` (https://github.com/dlang/phobos/blob/931a25126b4fabaca0ed261d145c9aa444a193f7/std/concurrency.d#L1904) and once in the List itself - it uses the GC for all allocation - the list implementation isn't very efficient The challenge in case someone is interested: https://github.com/Voultapher/Fast-Code-Competition

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