Changed raw pointer name from ptr to raw_pointer to avoid confusion with
the `use std::ptr` statement a few lines above. This way the crate name
and pointer name are well differenciated.

Add documentation example to slice_from_raw_parts_mut()
Add SAFETY explanations to some unsafe blocks in libcore/ptr

* libcore/ptr/mod.rs
* libcore/ptr/unique.rs
* libcore/ptr/non_null.rs

safety-mod.rs: Add SAFETY to slice_from_raw_parts(),
slice_from_raw_parts_mut()

slice_from_raw_parts_mut: Add documentation example

safety-ptr-unique.rs: Add SAFETY to new() and cast()

safety-ptr-non_null.rs: Add SAFETY to new()

safety-ptr-non_null.rs: Add SAFETY to cast()

safety-ptr-non_null.rs: Add SAFETY to from() impls

safety-ptr-unique.rs: Add SAFETY to from() impls

safety-ptr-non_null.rs: Add SAFETY to new()

safety-ptr-unique.rs: Add SAFETY to new()

safety-ptr-mod.rs: Fix safety explanation

rust-lang#71507 (comment)

safety-prt-non_null.rs: Fix SAFETY comment syntax

safety-ptr-unique.rs: Fix syntax for empty()

safety-ptr-non_null.rs: Fix misuse of non-null for align_of()

safety-ptr-non_null.rs: Remove incorrect SAFETY comment

safety-ptr-unique.rs: Remove unsound SAFETY comment

safety-ptr-mod.rs: Add std comment on slice_from_raw_parts guarantee

safety-ptr-unique.rs: Remove incorrect safety comment

Creating a Unique from a NonNull has strict guarantees that the current
implementation does not guarantee

rust-lang#71507 (comment)

safety-ptr: Re-adding ignore-tidy directive