work around pointless copy in isZeroMemory

[tersec]

For example, given

import stew/objects
discard isZeroMemory(default(array[1024, byte]))

The previous code under Nim 2.0.8 with refc compiles to:

N_LIB_PRIVATE N_NIMCALL(NIM_BOOL,
                        _ZN7objects12isZeroMemoryE5arrayI10range010235uInt8E)(
    tyArray__P46RzO9bG1lpfJxQ2itJfAA x_p0) {
  NIM_BOOL result;
  {
    result = (NIM_BOOL)0;
    {
      NU8 b;
      tyArray__P46RzO9bG1lpfJxQ2itJfAA colontmp_;  /* stack copy of the array */
      NI i;
      b = (NU8)0;
      nimZeroMem((void *)colontmp_, sizeof(tyArray__P46RzO9bG1lpfJxQ2itJfAA));
      nimCopyMem((void *)colontmp_, (NIM_CONST void *)((NU8 *)(x_p0)),
                 sizeof(tyArray__P46RzO9bG1lpfJxQ2itJfAA));
      i = ((NI)0);
      {
        while (1) {
          b = colontmp_[(i)-0];
          {
            if (!!((b == ((NU8)0))))
              goto LA6_;
            result = NIM_FALSE;
            goto BeforeRet_;
          }
        LA6_:;
          {
            if (!(((NI)1023) <= ((NI)(i))))
              goto LA10_;
            goto LA2;
          }
        LA10_:;
          i += ((NI)1);
        }
      }
    LA2:;
    }
    result = NIM_TRUE;
  }
BeforeRet_:;
  popFrame();
  return result;
}

which this PR changes to codegen to

N_LIB_PRIVATE N_NIMCALL(NIM_BOOL,
                        _ZN7objects12isZeroMemoryE5arrayI10range010235uInt8E)(
    tyArray__P46RzO9bG1lpfJxQ2itJfAA x_p0) {
  NIM_BOOL result;
  NU8 *bufPtr;
  {
    result = (NIM_BOOL)0;
    bufPtr = ((NU8 *)(x_p0));
    {
      NU8 b;
      NI i;
      b = (NU8)0;
      i = ((NI)0);
      {
        while (1) {
          b = bufPtr[(i)-0];
          {
            if (!!((b == ((NU8)0))))
              goto LA6_;
            result = NIM_FALSE;
            goto BeforeRet_;
          }
        LA6_:;
          {
            if (!(((NI)1023) <= ((NI)(i))))
              goto LA10_;
            goto LA2;
          }
        LA10_:;
          i += ((NI)1);
        }
      }
    LA2:;
    }
    result = NIM_TRUE;
  }
BeforeRet_:;
  popFrame();
  return result;
}

which as intended does not trigger a copy of isZeroMemory's parameter.

initHashSet unconditionally exists in std/sets since at the latest 1.6.0: https://github.com/nim-lang/Nim/blob/v1.6.0/lib/pure/collections/sets.nim#L112-L129

default unconditionally also unconditionally exists since at the latest 1.6.0: https://github.com/nim-lang/Nim/blob/v1.6.0/lib/system.nim#L982-L998