Shade jawn-parser

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Followup to #655.

We have a binary incompatibility between jawn-parser 0.13.0, brought in transitively by github4s 0.19.0, and jawn-parser 0.10.4, brought in by sbt 1.2.6. The latter gets added to the classpath, and arbitrarily selected, no matter what dependency exclusions we might define in the project. Thus we should use https://github.com/sbt/sbt-assembly to shade our usage of the parser in circe-jawn 0.10.0, and make this work with our released plugin.

See also eed3si9n/sjson-new#100, prompted by sbt/serialization#21,