refactor rune-cli to be more command oriented

[tgolsson]

This PR changes rune (rune-cli) to be based around commands and feel a bit more like cargo.

In this PR there is now three commands for Rune:

• check: only compiles, emites all errors and warnings
• test: runs all tests
• run: runs the main entrypoint

Furthermore, there is now auto-find for the entrypoint as well, looking for main.rn, lib.rn, as well as in the subfolders src and script. This was necessary as one would otherwise have to make "path" part of every subcommand, which makes everything more complicated while being less ergonomic and needing more typing. The old mode is still possible by passing paths before the command.

Sample output:

➜ ../rune/target/debug/rune check
Checking: script/main.rn
// snip a ton of badly formatted warnings

➜ ../rune/target/debug/rune test
Found 23 tests...
Test day1::test_part1               passed
Test day1::test_part2               passed
Test day2::test_part1               passed
Test day2::test_part2               passed
Test day3::test_part1               passed
Test day3::test_part2               passed
Test day8::test_part1               passed
Test day8::test_part2               passed
Test day9::part2                    passed
Test day9::part1                    passed
Test day10::test_part1              passed
Test day10::test_part2              passed
Test day12::test_part1              passed
Test day12::test_part2              passed
Test day12::test_part2_rots         passed
Test day12::test_part2_ss           passed
Test day11::test_part1              passed
Test day11::test_part2              passed
Test day13::test_part1              passed
Test day13::test_part2              passed
Test day14::test_part1              passed
Test day14::test_part2              passed
Test day14::test_parse              passed
====
Executed 23 tests with 0 failures (0 skipped) in 0.308 seconds

➜ ../rune/target/debug/rune --color=always test -q
Found 23 tests...
.......................
====
Executed 23 tests with 0 failures (0 skipped) in 0.308 seconds

➜ ../rune/target/debug/rune run
== () (5.130348676s)

[udoprog]

Added a few things:

• Added SharedArgs, so that paths can be specified inside of each subcommand.
• Cleaned up context and options construction a bit, since they should be done the same everywhere and fully depend on the args.
• Modified the book to use the new run subcommand (hopefully didn't miss anything).

[udoprog]

Assuming you're OK with my changes, feel free to merge.

[udoprog]

This is the only part I'm a little bit skeptical over right now. All the entrypoints seem like potentially surprising, and they depend on where the command is executed rather than a well-defined project root.

I'm fine with keeping them for now though (maybe consider trimming) and superseding them with a more explicit mechanism in the future.

[udoprog]

Thanks for the overhaul!

It was a bit awkward to specify the paths before the subcommand, so I collapsed them into a SharedArgs which is flattened into each relevant subcommand instead.