0.21.0: negative array slicing

[He-Pin]

std.slice([0,1,2,3], -1, null, null) should give [3] but get [0,1,2,3] in sjsonnet

refs: google/jsonnet#1222 (comment)

refs: google/jsonnet#1093
refs: CertainLach/jrsonnet#117

  slice(indexable, index, end, step)::
    local invar =
      // loop invariant with defaults applied
      {
        indexable: indexable,
        index:
          if index == null
          then 0
          else
            if index < 0
            then std.max(0, std.length(indexable) + index)
            else index,
        end:
          if end == null
          then std.length(indexable)
          else
            if end < 0
            then std.length(indexable) + end
            else end,
        step:
          if step == null
          then 1
          else step,
        length: std.length(indexable),
        type: std.type(indexable),
      };
    assert invar.step >= 0 : 'got [%s:%s:%s] but negative steps are not supported' % [invar.index, invar.end, invar.step];
    assert step != 0 : 'got %s but step must be greater than 0' % step;
    assert std.isString(indexable) || std.isArray(indexable) : 'std.slice accepts a string or an array, but got: %s' % std.type(indexable);
    local build(slice, cur) =
      if cur >= invar.end || cur >= invar.length then
        slice
      else
        build(
          if invar.type == 'string' then
            slice + invar.indexable[cur]
          else
            slice + [invar.indexable[cur]],
          cur + invar.step
        ) tailstrict;
    build(if invar.type == 'string' then '' else [], invar.index),