Rename _copyto! as copyto_layout!

[dlfivefifty]

I think a more descriptive name would be good here.

[jishnub]

Bikeshedding, but should this be layout_copyto! to be consistent with layout_getindex? Personally, I prefer this function_layout naming style, but renaming layout_getindex might require considerable changes across the ecosystem.

[dlfivefifty]

I don't think its considerable changes to do layout_getindex -> getindex_layout since thats only ever called, never overloaded.