Rollup merge of rust-lang#67163 - TheSamsa:split-up-ptr-mod, r=Mark-Simulacrum

Split up ptr/mod.rs in libcore...

...one with implementation detail for const ptr and the other with mut ptr

I am not sure if the "stable since 1.0.0" flags are the correct choice for the two additional mods.
Also, is it necessary for them to be "pub"? If so, there should be a good description for them.

Closes rust-lang#66891

Author: Centril