Move all the simple bool query methods (is_table_inside,
is_table_cell, is_none, is_block_outside, etc.) from Display.cpp into
Display.h as inline definitions. These are trivial one-liners called
in very hot painting and layout paths.
Keep only to_string() and from_short() out-of-line.
The spec just says to follow "most backwards-compatible, then shortest"
when serializing these (and it does so in a very hand-wavy fashion).
By omitting some keywords when they are implied, we end up matching
other engines and pass a bunch of WPT tests.
