Optimize Keyword.validate/2 to avoid building new list

[sabiwara]

Maybe I'm missing something, but as far as I can tell, there is no need to re-build that kw-list, since in the case there is a "bad_key" we're not using the acc anyway?
So we just end up reversing the list for no reason - this new version just returns the original as is, which is less work:

# main
iex> Keyword.validate!([a: 1, b: 2], [:a, :b])
[b: 2, a: 1]
# this branch
iex> Keyword.validate!([a: 1, b: 2], [:a, :b])
[a: 1, b: 2]

Confirmed with a quick benchmark that this reduces the memory footprint:

Comparison: 
candidate        7.63 M
original         6.76 M - 1.13x slower +16.80 ns

Memory usage statistics:

Name         Memory usage
candidate           264 B
original            424 B - 1.61x memory usage +160 B