This updates CargoTool to subclass the newer BaseTool, and to use

the newer features it offers (dependencies, improved file extension

handling, centralized base command building, and new comment options).
The parsing has been completely redone. This tool runs both

cargo clippy and cargo test, and these output content differently.

For cargo clippy, we can safely use --message-format=json to get a

series of JSON payloads we can process, which works well, and is less

error-prone than the previous parsing logic.
However, JSON output is, as of now, only available in nightly builds of

cargo test, so we can't depend on it. Instead, we use pattern matching

to try to find the test output, turning that into a general comment.

It's still better than the old logic, which as far as I can tell no

longer works (or maybe never did), since the old logic was looking for

output that isn't present in at least modern builds of cargo test.
There's also some additional handling for unit tests. If cargo clippy

is enabled, it will be run against unit tests, using the (semi-modern

--tests argument). If not, and cargo test encounters a syntax error,

it will be reported, along with recommendations.
Test output is also capped to 100 lines.
Unit tests cover all these situations, and we have notes to support

future JSON output from cargo clippy.
This is the last tool that needed to be ported over to the new tool

and test support.

Unit tests pass on Python 2.7 and 3.x.