remove duplicate function from std::ptr (is_null, is_not_null, offset, mut_offset)
auto merge of #12167 : JeremyLetang/rust/ptr-func, r=alexcrichton

This pull request remove the functions `std::ptr::{offset, mut_offset, is_null, is_not_null}`, which are duplicate with the corresponding methods in `std::ptr::RawPtr`.

I don't know if these changes are acceptable / needed, but if it is, maybe we can remove `std::ptr::{null, mut_null}`, to use only `std::ptr::RawPtr::null`