Switch from special type names (sexp_{list,bool,array,option}) to attributes

[copy]

Opening Core_kernel or Base bring the type aliases sexp_{list,bool,array,option} to {list,bool,array,option} into scope, which infamously infect OCaml tooling (merlin, utop) and confuse newcomers. See also janestreet/base#39.

ppx_sexp_conv already supports attributes that supersede the old type names. So we should encourage users to switch to attributes and fix the original problem by moving the type aliases out of Core_kernel and Base (maybe to a submodule?).

[ghost]

Indeed, we have been talking about doing this change for a while. It's just that we never took the time to do it.

Regarding moving the types aliases to a sub-module, for the user this is basically as much work as switching to attributes. We could mark them deprecated with [@@deprecated] though.

[copy]

It would be wonderful if 2019 could be the year in which we don't have to see sexp_list any more. The more one uses merlin, the more often they come up, which is quite unfortunate.

[yminsky]

Yeah, it's pretty annoying. It's on our near-term todo list.

[kevinji]

See b415c54!

[seliopou]

These have been deprecated for a while in base and core_kernel, so I'm gonna go ahead and delete them entirely.