Function type_decl_to_ocaml_decl 

pub(crate) fn type_decl_to_ocaml_decl(
    ctx: &GenerateCtx<'_>,
    decl: &TypeDecl,
    co_rec: bool,
) -> String

Generate an ocaml type declaration that mirrors decl.

co_rec indicates whether this definition is co-recursive with the ones that come before (i.e. should be declared with and instead of type).