`t.TypeOf<keyof<o>>` should return a string union type, not a numeric union type.

[noshiro-pf]

🐛 Bug report

Current Behavior

import * as t from "io-ts";

const T = t.keyof({
  0: undefined,
  1: undefined,
  2: undefined,
  3: undefined,
  4: undefined,
});

type T = t.TypeOf<typeof T>;  // 0 | 1 | 2 | 3 | 4

console.log(T.decode(0));

{
    "_tag": "Left",
    "left": [
        {
            "value": 0,
            "context": [
                {
                    "key": "",
                    "type": {
                        "name": "\"0\" | \"1\" | \"2\" | \"3\" | \"4\"",
                        "keys": {},
                        "_tag": "KeyofType"
                    },
                    "actual": 0
                }
            ]
        }
    ]
}

console.log(T.decode("0"));

{
    "_tag": "Right",
    "right": "0"
}

Expected behavior

import * as t from "io-ts";

const T = t.keyof({
  0: undefined,
  1: undefined,
  2: undefined,
  3: undefined,
  4: undefined,
});

type T = t.TypeOf<typeof T>;  // "0" | "1" | "2" | "3" | "4"

Runtime behavior is ok.

It would be nice if there was some way to support the union of literals trick for numeric literals.

Reproducible example

Suggested solution(s)

Additional context

Your environment

Which versions of io-ts are affected by this issue? Did this work in previous versions of io-ts?

Software	Version(s)
io-ts	2.2.20
fp-ts	2.13.1
TypeScript	4.9.3