Add slice::from_pointer_range

[khuey]

#65806 added [T]::as_ptr_range. It would be nice if there were a slice::from_raw_parts analog that went the other way. Range<*const T> is a convenient way to pass around the start and length information needed for a slice.

[KonaeAkira]

#89792 exists. Can this issue be closed?

[khuey]

Yeah.