You can *only* do unaligned SIMD on modern x86, and not all the operations support it. You certainly can't do it on ARM. If we support specifying alignment for types and SIMD types, then we need to handle it in the heap allocator.

> We make up for it by bundling an allocator, but that's appearing to be unacceptable for some distributions (perhaps). 

Neither Fedora or Debian will care if we use jemalloc's API. The issue is that they want a way to use an external jemalloc, for example by passing `--external-jemalloc` to `configure`. They'll also expect us to do this with libuv and LLVM, and I expect whatever infrastructure we need for checking if the system versions are okay will take care of all 3.