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.
CSS::Display::Type and CSS::Display::ListItem defaulted to int (4
bytes each). Give them explicit u8 underlying types, shrinking
CSS::Display from 12 bytes to 4 bytes.