Fix move operators

[depau]

This PR addresses the following:

• Some move operators had the following check:

  if (this != &other) return *this;

  which is obviously a typo since you want to return this if it's the same, not if it's different.

• (not really important) while I was there I also added noexcept to allow move operators to be used by the STL containers. See here for an explanation why

I highly recommend you try clangd and/or clang-tidy: I see in v4.0.0 you additionally have some missing returns in move ops, clangd would have easily caught it.

[RobLoach]

Thanks so much for this. Good catches. I'll have to check out clangd!