Return an optional for numeric conversions instead of trapping?

[rvanasa]

Instead of trapping for lossy numeric conversions (e.g. Nat8.toNat() or Int.toNat()), it could make sense to return an optional value to give the developer a way to handle the error case in a more ergonomic way.

Examples:

let ?n = Int.toNat(-5) else return #err("Custom error message");

assert switch (Int.toNat(5)) {
  case (?n) n == 5;
  case _ false
}

For developers who want to maximize performance, or in cases where you can guarantee that the number will convert without trapping, it would remain possible to bypass this check by directly calling the corresponding Prim function.

Currently, Int.toNat() / Nat.fromInt() return an optional while fixed-size numeric types trap. Therefore, we should change the conversions one way or the other for consistency.

[crusso]

Users shouldn't use Prim directly - I wish we could lock it down..
May we should add a second layer of conversion
Nat8.fromNat : Nat -> Nat8
Nat8.asNat : Nat -> ?Nat8
to avoid breaking too much old code that uses the trapping versions
(Names TBD)

[rvanasa]

Resolved by #199.

[rvanasa]

Reopening in case we want to revisit this (CC @ggreif).