Don't preserve newlines in attributes

[k0kubun]

Haml::Helpers.preserve is very slow. I don't think it's necessary for attributes because Slim, Faml and Hamlit don't preserve them and work fine.

The original change was done in 2008 e8075e7 and no reason is found to keep that change. Let's remove a legacy feature.

Benchmark

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

before

$ bundle exec ruby bench.rb
Rendering: /home/k0kubun/src/github.com/k0kubun/haml_bench/templates/slim_bench.haml
Calculating -------------------------------------
          haml 4.0.7     3.576k i/100ms
   haml 5.0.0.beta.2     5.450k i/100ms
-------------------------------------------------
          haml 4.0.7     37.559k (± 3.6%) i/s -    189.528k
   haml 5.0.0.beta.2     55.625k (± 4.5%) i/s -    277.950k

Comparison:
   haml 5.0.0.beta.2:    55625.3 i/s
          haml 4.0.7:    37558.8 i/s - 1.48x slower

after

$ bundle exec ruby bench.rb
Rendering: /home/k0kubun/src/github.com/k0kubun/haml_bench/templates/slim_bench.haml
Calculating -------------------------------------
          haml 4.0.7     3.608k i/100ms
   haml 5.0.0.beta.2     6.140k i/100ms
-------------------------------------------------
          haml 4.0.7     37.460k (± 3.5%) i/s -    187.616k
   haml 5.0.0.beta.2     64.248k (± 4.5%) i/s -    325.420k

Comparison:
   haml 5.0.0.beta.2:    64247.5 i/s
          haml 4.0.7:    37459.9 i/s - 1.72x slower

@haml/committers WDYT?

[k0kubun]

I finished attribute compiler implementation using this. Merging for now. Please give me your opinions later.