[Issue 18899] destroy is inefficient for small structs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18899

--- Comment #3 from Mike Franklin <slavo5150 at yahoo.com> ---
Yes, `dest[] = src[]` gets lowered to _d_arraycopy, which ultimately calls
memcpy

View ASM here:  https://run.dlang.io/is/izCLp0
_d_arraycopy implementation here: 
https://github.com/dlang/druntime/blob/2db828bd4f21807254b770b3ec304f14596a9805/src/rt/arraycat.d#L22-L29

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