Alternate to nim-lang#15915 (nim-lang#15937)

* Alternate PR to nim-lang#15915 to
resolve the problem mentioned there (`hash() == 0`) as well as
to close nim-lang#15624

* Address nim-lang#15937 (comment)
{ though this was only a move from 2 copies to 3 copies. ;-) }

lib/pure/hashes.nim

@@ -113,8 +113,12 @@ proc hashWangYi1*(x: int64|uint64|Hash): Hash {.inline.} =
   const P0  = 0xa0761d6478bd642f'u64
   const P1  = 0xe7037ed1a0b428db'u64
   const P58 = 0xeb44accab455d165'u64 xor 8'u64
+  template h(x): untyped = hiXorLo(hiXorLo(P0, uint64(x) xor P1), P58)
   when nimvm:
-    cast[Hash](hiXorLo(hiXorLo(P0, uint64(x) xor P1), P58))
+    when defined(js): # Nim int64<->JS Number & VM match => JS gets 32-bit hash
+      result = cast[Hash](h(x)) and cast[Hash](0xFFFFFFFF)
+    else:
+      result = cast[Hash](h(x))
   else:
     when defined(js):
       asm """
@@ -132,8 +136,9 @@ proc hashWangYi1*(x: int64|uint64|Hash): Hash {.inline.} =
           var res   = hi_xor_lo_js(hi_xor_lo_js(P0, BigInt(`x`) ^ P1), P58);
           `result`  = Number(res & ((BigInt(1) << BigInt(53)) - BigInt(1)));
         }"""
+      result = result and cast[Hash](0xFFFFFFFF)
     else:
-      cast[Hash](hiXorLo(hiXorLo(P0, uint64(x) xor P1), P58))
+      result = cast[Hash](h(x))
 
 proc hashData*(data: pointer, size: int): Hash =
   ## Hashes an array of bytes of size `size`.