Notable changes

• Fixed a typo when parsing JSON of head table;
• The consolidator will no longer produce incorrect lookup IDs when the quantity of lookups are changed.
• The heuristics of turning a lookup into an extension lookup is relaxed, in order to hold larger single-subs.
• More heuristics are added to the building step, especially when building GSUB table, to avoid malfunction in legacy environments. Currently a heuristics is applied to the vert feature.