User friendly and more functional mkunion

V1.2.x

• Add breadth-first reducer traversal

V1.3.x

• Use go:embed for templates

V1.4.x

• Add function generation like Match to simplify work with unions
• Benchmark implementation of Match vs Reducer (depth-first has close performance, but breadth-first is much slower)

V1.5.x

• Add support for multiple go:generate mkunion in one file

V1.6.x

• Add variant types inference