Fix textareas only after preserved timing

[k0kubun]

Description

We call Haml::Buffer#fix_textareas! to rollback textarea content's first 
, which is unexpectedly preserved from \n by Haml::Helpers#preserve, to \n for every script.

But it means that we need to call Haml::Buffer#fix_textareas! only after we call preserve. So I fixed to do so.

Benchmark

With Ruby 2.4.0 and k0kubun/haml_bench/templates/slim_bench.haml,

before

Calculating -------------------------------------
          haml 4.0.7     41.534k (± 1.4%) i/s -    210.652k in   5.072717s
          haml 5.0.1    138.067k (± 1.5%) i/s -    700.872k in   5.077348s

Comparison:
          haml 5.0.1:   138067.3 i/s
          haml 4.0.7:    41534.1 i/s - 3.32x  slower

after

Calculating -------------------------------------
          haml 4.0.7     39.466k (± 2.1%) i/s -    199.206k in   5.049870s
          haml 5.0.1    148.617k (± 1.9%) i/s -    750.109k in   5.049201s

Comparison:
          haml 5.0.1:   148616.6 i/s
          haml 4.0.7:    39466.1 i/s - 3.77x  slower

Now _hamlout's dependency is only _hamlout.attributes call for Hash in that benchmark template. It means that we have new optimization chance to lazily initialize Haml::Buffer instance, which is the current bottleneck of Haml.

[k0kubun]

Note that toplevel? is considered to be added just for optimization, and it must be removed now to apply this in partial view (we didn't need to call in partial before because outside layout will render partial as ruby script and call fix_textareas! for it).

And this fix won't cause regression as this method is idempotent and safe to call multiple times.

[k0kubun]

As the original problem is heavily tested for both Rails and non-Rails templates and tests pass, I think this is safe to merge.

@haml/committers Thoughts?

[k0kubun]

Since this patch does not introduce any behavior change, merging this for now. Please fell free to add some feedback later.