Use accumulator on raw_html

[ypconstante]

Today raw_html creates a new list wrapping up the sibling current html value before adding the current node data, causing a small memory overhead.

This PR moves raw_html to use an accumulator, simplified build_raw_html a bit by always expecting a list and merged tag_for into the only build_raw_html that was using it.

##### With input big #####
Name                   ips        average  deviation         median         99th %
bench (pr)           48.88       20.46 ms     ±6.66%       20.37 ms       25.04 ms
bench (main)         43.07       23.22 ms    ±15.12%       22.69 ms       30.49 ms

Comparison:
bench (pr)           48.88
bench (main)         43.07 - 1.13x slower +2.76 ms

Memory usage statistics:

Name            Memory usage
bench (pr)          12.12 MB
bench (main)        12.50 MB - 1.03x memory usage +0.39 MB

**All measurements for memory usage were the same**

##### With input medium #####
Name                   ips        average  deviation         median         99th %
bench (pr)          219.61        4.55 ms     ±6.63%        4.47 ms        5.58 ms
bench (main)        141.74        7.06 ms    ±20.26%        6.46 ms       10.27 ms

Comparison:
bench (pr)          219.61
bench (main)        141.74 - 1.55x slower +2.50 ms

Memory usage statistics:

Name            Memory usage
bench (pr)           3.77 MB
bench (main)         4.08 MB - 1.08x memory usage +0.30 MB

**All measurements for memory usage were the same**

##### With input small #####
Name                   ips        average  deviation         median         99th %
bench (pr)          955.98        1.05 ms    ±19.26%        1.06 ms        1.59 ms
bench (main)        732.85        1.36 ms    ±24.18%        1.50 ms        2.10 ms

Comparison:
bench (pr)          955.98
bench (main)        732.85 - 1.30x slower +0.32 ms

Memory usage statistics:

Name            Memory usage
bench (pr)         774.63 KB
bench (main)       877.17 KB - 1.13x memory usage +102.54 KB

[philss]

Nice! Thank you!

I added a minor suggestion. Not sure if it will make any difference in memory.