Revert "don't use the deprecated `Utf8String` ref type"
Revert "don't use the deprecated `Utf8String` ref type"

It's still available in `ref`.

This reverts commit 90f033e9d13f4111d4be9942ab1fc67403100ff6.

unbornchikken/node-ffi,skoky/node-ffi,paulcbetts/node-ffi,lzpfmh/node-ffi,paulcbetts/node-ffi,lzpfmh/node-ffi,unrealsolver/node-ffi,pmuellr/node-ffi,craigcomstocks/node-ffi,node-ffi/node-ffi,Icenium/node-ffi,unbornchikken/node-ffi,craigcomstocks/node-ffi,lzpfmh/node-ffi,unbornchikken/node-ffi,craigcomstocks/node-ffi,skoky/node-ffi,paulcbetts/node-ffi,pmuellr/node-ffi,pmuellr/node-ffi,Icenium/node-ffi,node-ffi/node-ffi,paulcbetts/node-ffi,skoky/node-ffi,node-ffi/node-ffi,pmuellr/node-ffi,unrealsolver/node-ffi,lzpfmh/node-ffi,unrealsolver/node-ffi,node-ffi/node-ffi,craigcomstocks/node-ffi,skoky/node-ffi,unrealsolver/node-ffi,Icenium/node-ffi,unbornchikken/node-ffi,Icenium/node-ffi