* Rename MkdirOption interface to MkdirOptions
It was hard to remember which Options interfaces were spelled in the
singular and which in the plural (and anyway this one contained two
options).
Also added MkdirOptions to cli/js/deno.ts. All the analogous interfaces
were exported there.
* Rename RemoveOption interface to RemoveOptions
This was the last remaining Option interface spelled in the singular.
Easier to remember if they're all Option**s**; plus we may want to add extra
options here later.
* 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.
