Rollup merge of #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 #66891

Author: Centril