refactor hash in JS backend

[ringabout]

make hash more type-safe in JS backend

• Next PR, I will make hash(x: float) works in safari browser and IE 11.
  Ref fix #16542 #16549 (comment)

The workaround which I discussed with @timotheecour long time ago.

      const buffer = new ArrayBuffer(8);
      const floatBuffer = new Float64Array(buffer);
      const uintBuffer = new Uint32Array(buffer);
      floatBuffer[0] = `x`;
      `result` = BigInt(uintBuffer[0]) + (BigInt(uintBuffer[1]) << 32n);

[timotheecour]

LGTM again.

these performance tests confirm what I was suspecting:

import std/jsbigints
import std/times

type Hash = int

proc fn(a: JsBigInt) =
  when defined case1:
    let maxSafeInteger = big"0x1FFFFFFFFFFFFF"
    let a1 = cast[Hash](toNumber((a and maxSafeInteger))) and cast[Hash](0xFFFFFFFF)
    let a2 = cast[Hash](toNumber(wrapToInt(a, 32)))
    doAssert a1 == a2

  elif defined case2:
    let maxSafeInteger = big"0x1FFFFFFFFFFFFF"
    let a1 = cast[Hash](toNumber((a and maxSafeInteger))) and cast[Hash](0xFFFFFFFF)
    doAssert a1 != 12

  elif defined case3:
    let maxSafeInteger = (big"1" shl big"53") - big"1"
    let a1 = cast[Hash](toNumber((a and maxSafeInteger))) and cast[Hash](0xFFFFFFFF)
    doAssert a1 != 12

  elif defined case4:
    let a1 = cast[Hash](toNumber(wrapToInt(a, 32)))
    doAssert a1 != 12
  else: static doAssert false

proc main()=
  let t = now()
  for i in 123..<10_000_000:
    fn(big(i))
  let t2 = now() - t
  echo t2
main()

nim r -b:js -d:danger -d:case1 main
2 seconds and 320 milliseconds
nim r -b:js -d:danger -d:case2 main
2 seconds and 73 milliseconds
nim r -b:js -d:danger -d:case3 main
4 seconds and 104 milliseconds
nim r -b:js -d:danger -d:case4 main
538 milliseconds

conclusion 1: let maxSafeInteger = (big"1" shl big"53") - big"1" isn't inlined nor optimized, so using big"0x1FFFFFFFFFFFFF" directly is best
conclusion 2: wrapToInt is indeed faster (and gives same results)

(EDIT) conclusion 3: so this PR might improve performance of hash on js, but i haven't measured that