Add Jil to JsonSerializersBenchmark

Adds a dependency on Sigil 4.0.2 (https://github.com/kevin-montrose/Sigil) and Jil 1.0.1 (https://github.com/kevin-montrose/Jil); both are available in Nuget.

Jil is quite fast on my machine, but not as fast as protobuf-net.