core: Inline the protocol definitions of sendp and oneshot

I want to be able to see what is going on
core: Remove a stray #[doc(hidden)] attribute
core::comm: Remove functions in favor of methods

`send`, etc. are never used. I've left the functions for oneshot pipes
because by-value methods don't work.
core::pipes: Just reordering declarations
core::comm: Modernize constructors no use `new`