This library provides two new integer types, Optint.t and Int63.t, which
guarantee efficient representation on 64-bit architectures and provide a
best-effort boxed representation on 32-bit architectures.

Goal

The standard Int32.t and Int64.t types provided by the standard library have
the same heap-allocated representation on all architectures. This consistent
representation has costs in both memory and run-time performance.

On 64-bit architectures, it's often more efficient to use the native int
directly.
This library provides types to do exactly this:

• Optint.t: an integer containing at least 32 bits. On 64-bit, this is an
  immediate integer; on 32-bit, it is a boxed 32-bit value. The overflow
  behaviour is platform-dependent.

• Int63.t: an integer containing exactly 63 bits. On 64-bit, this is an
  immediate integer; on 32-bit, it is a boxed 64-bit integer that is wrapped to
  provide 63-bit two's complement semantics. The two implementations are
  observationally equivalent, modulo use of Marshal and Obj.

In summary:

| Integer type | 32-bit representation | 64-bit representation | Semantics |
| -- | -- | -- | -- |
| Stdlib.Int.t | 31-bit immediate ✅ | 63-bit immediate ✅ | Always immediate |
| Stdlib.Nativeint.t | 32-bit boxed ❌ | 64-bit boxed ❌ | Exactly word size |
| Stdlib.Int32.t | 32-bit boxed ❌ | 32-bit boxed ❌ | Exactly 32 bits |
| Stdlib.Int64.t | 64-bit boxed ❌ | 64-bit boxed ❌ | Exactly 64 bits |
| Optint.t (new) | 32-bit boxed ❌ | 63-bit immediate ✅ | At least 32 bits |
| Int63.t (new) | 64-bit boxed ❌ | 63-bit immediate ✅ | Exactly 63 bits |

These new types are safe and well-tested, but their architecture-dependent
implementation makes them unsuitable for use with the Marshal module. Use the
provided encode and decode functions instead.