Macro code coverage tool

[Blacksmoke16]

Intro

This PR represents an initial implementation of a new compiler tool to generate a https://docs.codecov.com/docs/codecov-custom-coverage-format coverage report for macro code. Functionally, the tool feels very robust, with a ton of spec coverage. I tested it out on the various macro code within Athena and I'm at the point now where I'm satisfied with how it's working to the point where I feel comfortable opening this PR.

I've created the PR as a draft as it depends on some other PRs, and because it intentionally includes debug code commented out. The intent/goal around this PR is to mainly show off the implementation thus far, and to solicit feedback either in the form of other changes that would make this PR's implementation simpler, or general high level tweaks.

Please go try it out on your own project(s) to see if you can find any problems/issues.

Implementation Overview

At a high level the current implementation adds a new crystal tool that when executed enables a new collect_covered_macro_nodes property on the program. When enabled, the macro interpreter will collect macro AST nodes via the #collect_covered_node method. Because a new macro interpreter is used for each expansion, the collected nodes are "flushed" to another array pending processing via the tool after each macro is expanded.

The tool itself iterates over each "group" of collected nodes from each expansion, skipping any that do not relate to user code. It then iterates over reach "group", chunking them based on the line number. The chunking ensures that each node can increment that line's count by 1 or 0, not how many nodes are on that single line.

From here, we then perform some heuristics to determine if a given line was a hit, partial hit, or a miss. Partial hits are kept track by caching the specific conditionals and if it was a hit/miss on a line so we can know how many paths of the total possibilities we hit on that single line.

Macro raise is handled by raising a new SkipMacroCodeCoverageException that extends SkipMacroException. Rescuing this exception allows the tool to run on the code that was collected up to before the exception was raised, while not resulting in an error/running all the other code after it. The exception's message/trace is printed to STDOUT while the report is written to STDERR. This is esp useful for testing error flows of custom macro logic as it allows the code to assert the proper error was raised, while still allowing to save the coverage report. I.e. the report JSON/exception outputs are not co-mingled. The main benefit of this is preventing the need to run these kind of tests twice, once for the coverage report and once for the actual test assertions.

Depends on #15709

Resolves #14880

Open Issues

Will keep a running list of additional things that need handled:

• CI failing on Crystal 1.0
• Non-user raised exceptions being swallowed

[Blacksmoke16]

Pushed up another commit to remove the debug code and such. Haven't heard any preliminary feedback yet, so going to mark this as ready for review!