Relax the generation of `t_n`

ts2ocaml generates types for all of the arrity possible of a given type.

However, in practice, the types are stricly equal and have the same arrity as the type t.

Additionnally, bindings generated seem to always use the type with highest arrity.

So I would propose that:

• We don’t generate types if they have the same arrity as t
• (optionnally), we only generate t as it seems compatible with how other generated bindings will use the module.

[cannorin]

Now we have an option --safe-arity to customize this behavior since https://github.com/ocsigen/ts2ocaml/commit/1f0ed1e771133424fed0ad855f0f58b9831128ca .

• --safe-arity=full (default) generates t_n types in the module, and tries to use t_n type if the type is unknown (e.g. from another package)
• --safe-arity=provide only generates t_n types in the module
• --safe-arity=consume only tries to use t_n type if the type is unknown
• --safe-arity=off disables any usage of t_n types

[cannorin]

The above was addressed in https://github.com/ocsigen/ts2ocaml/commit/22191ab2220cbfeb1b0d7244650195934c4eafc4 (in fact, --safe-arity=off didn’t generate necessary t_n types at all)