reflect: undeprecate StringHeader, SliceHeader

[rsc]

CL https://go-review.googlesource.com/c/go/+/401434 marked these as deprecated, but if you want to do something like extract the pointer inside a string or a slice, it is unclear what you should use instead. We need to have an available answer before we ship a release with them deprecated.

See also #53003.

[bcmills]

Within std we currently have the internal/unsafeheader package, which defines String and Slice structs, but they're only marginally safer and not very ergonomic either.

[dmitshur]

CC @ianlancetaylor.

[rsc]

For Go 1.19 it definitely seems like we should roll back the deprecation CL.

[gopherbot]

Change https://go.dev/cl/408815 mentions this issue: Revert "reflect: deprecate (Slice|String)Header"