Linter for unboxing OCaml types

There are few reasons not to unbox types that can be. So let's find them!

Unboxing is allowed for:

• records with a single field: type t = { f : u }
• variants with a single constructor with a single parameter:
  • type t = C of u
  • type t = C of { f : u }
• allowed for parametric types, GADTs
• disallowed if the field is mutable

Pros:

• smaller memory footprint
• faster accesses
• no type safety concession

Cons: usage of magic features

• physical equality

type t = C of string
let s = "blah"
let x = C s
let y = C s
x == y (* true if [t] is unboxed, false otherwise *)

• non-regression on
  • marshalling
  • hashing
• cyclic values

type t = C of t
let rec x = C x (* forbidden if [t] is unboxed, allowed otherwise *)

For cases where we want to avoid unboxing, let's have a special annotation [@@do_not_unbox "reason"].

Cc @raphael-proust @hhugo1 @vch9