Append in place for strings and arrays

It's faster.

Behavior should be unchanged: no function inputs were altered, only local variables.

Benchmark:

    Benchmark.bm(first_col_width) do |benchmark|
      benchmark.report('+= [0]') do
        a = []
        N.times { a += [0] }
      end

      benchmark.report('<< 0') do
        a = []
        N.times { a << 0 }
      end

      benchmark.report('<< [0, 1]') do
        a = []
        N.times { a += [0, 1] }
      end

      benchmark.report('push(*[0, 1])') do
        a = []
        N.times { a.push(*[0, 1]) }
      end

      benchmark.report('+= "ab"') do
        a = ''
        N.times { a += 'ab' }
      end

      benchmark.report('<< "ab"') do
        a = []
        N.times { a << 'ab' }
      end
    end

Result:

                        user     system      total        real
    += [0]             0.090000   0.010000   0.100000 (  0.093640)
    << 0               0.000000   0.000000   0.000000 (  0.000706)
    << [0, 1]          0.170000   0.000000   0.170000 (  0.164410)
    push(*[0, 1])      0.000000   0.000000   0.000000 (  0.001339)
    += "ab"            0.020000   0.000000   0.020000 (  0.024198)
    << "ab"            0.000000   0.000000   0.000000 (  0.001242)
Would it be possible to write it as:
`contents << tree.trees`

That way its more easy to understand.