MultiArrayList: use @memcpy as a workaround

Reverts bf642204b373e01314ecfb0c50a643dc4b05746f and uses a different
workaround, suggested by @LemonBoy.

There is either a compiler bug or a design flaw somewhere around here.
It does not have to block this branch, but I need to understand exactly
what's going on here and make it so that nobody ever has to run into
this problem again.
38441b5eabAndrew Kelley committed on 2/24/2021, 7:49:12 PMparent52c45bf
2 files changedLine totals unavailable