0.20.0

This release adds initial multi-threading support.

What's Changed

• Start migrating imported functions to the new definition style by @kateinoigakukun in #252
• Allocate JavaScriptEventLoop per thread in multi-threaded environment by @kateinoigakukun in #255
• Add WebWorkerTaskExecutor by @kateinoigakukun in #256

Full Changelog: 0.19.3...0.20.0