* rewrite test runner in Rust
* migrate "test" and "runTests" functions from std to "Deno" namespace
* use "Deno.test()" to run internal JS unit tests
* remove std downloads for Deno subcommands
- Exports diagnostic items from `diagnostics.ts` which are missing at
runtime.
- Returns an array of diagnostics, instead of an object with a property
of `items`. This is because of the way Rust deals with certain
structures, and shouldn't be exposed in the APIs.
