Revamp how fieldset and legend rendering is defined

Properly define the rendering of the fieldset and legend elements.

The layout model used is most similar to Gecko, which uses an anonymous box to hold the fieldset's contents.

Fixes whatwg#3955, fixes whatwg#3930, fixes whatwg#3929, fixes whatwg#3928, fixes whatwg#3927, fixes whatwg#3915, fixes whatwg#3913, fixes whatwg#3660, fixes whatwg#3331, fixes whatwg#2756, fixes whatwg#4013.

Tests:
https://github.com/web-platform-tests/wpt/tree/master/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements
https://github.com/web-platform-tests/wpt/tree/master/html/semantics/forms/the-fieldset-element