Separate safe and unsafe operations

There are some useful little utilities in your `builtin` package. However, some people might prefer to avoid unsafe operations, especially if they are not obviously unsafe.

I suggest that you create a `builtin/unsafe` sub-package that has all the functions that use the standard `unsafe` package. Retain in the main builtin package all those functions that don't use `unsafe`.

Start by reviewing the builtin package and identifying which functions import or use Go's standard unsafe package. Move those functions into builtin/unsafe while retaining the others in builtin. Done means the package separation is complete and the existing utility behavior remains available from the appropriate package.

Written by the indexing model from the issue text.