code: function arraySpliceIn(mutate, at, v, arr) {
      var len = arr.length;
      if (mutate) {
          var _i = len;
          while (_i >= at) {
              arr[_i--] = arr[_i];
          }arr[at] = v;
          return arr;
      }
      var i = 0,
          g = 0;
      var out = new Array(len + 1);
      while (i < at) {
          out[g++] = arr[i++];
      }out[at] = v;
      while (i < len) {
          out[++g] = arr[i++];
      }return out;
  }
docs: Insert a value into an array.
  
  @param mutate Should the input array be mutated?
  @param at Index to insert at.
  @param v Value to insert,
  @param arr Array.