Forward port CommandLineParser rewrite

[som-snytt]

Tests to follow. (When I learn how to add tests.)

Bookmarking:
scala/scala#6622
scala/scala#5882

[dottybot]

Hello, and thank you for opening this PR! 🎉

All contributors have signed the CLA, thank you! ❤️

Have an awesome day! ☀️

[smarter]

Can you add a link to the corresponding scala/scala PR or commit in the commit message ?

[nicolasstucki]

Could you add the links to the commit message (not only the PR description). It is to make sure it is in the git history.

[som-snytt]

Yes, I'll update. I just looked at this again, wondering whether to improve the quick'n'dirty code.

[som-snytt]

Added test and dependency on scala-testkit.

[som-snytt]

Probably spurious?

[error] scala.reflect.internal.FatalError: package object runtime does not have a decl currentMirror

[som-snytt]

I guess not spurious.

[error] error while loading package, class file '/root/.cache/coursier/v1/https/scala-webapps.epfl.ch/artifactory/central/org/scala-lang/scala-reflect/2.13.1/scala-reflect-2.13.1.jar(scala/reflect/macros/package.class)' is broken
[error] (class java.lang.RuntimeException/error reading Scala signature of package.class: Scala signature package has wrong version
[error]  expected: 5.0
[error]  found: 5.2 in package.class)

[som-snytt]

Removed testkit dependency, converted syntax, rebased.

[smarter]

Otherwise LGTM!

[smarter]

Why is that type ascription needed? Could it be moved to be the result type of cur instead?

Suggested change

	inline def cur = if done then EOF else line.charAt(pos): Int
	inline def cur: Int = if done then EOF else line.charAt(pos)

[som-snytt]

charAt is a char. The benefits of this style choice are to point out the problematic expression; it looks more cool; in result type position, it's like having eyeglasses wrapped in tape where they broke. That's why implicits, the nerdiest feature, requires it.

[smarter]

ok, I guess that's fine.

[som-snytt]

It's a bit quirky, and it was so long ago, I had to reconstruct my thinking. On the up side, with all the talk about command line parsing support for main entry point, probably this code will be replaced anyway. It was mostly aimed at Process building.

[som-snytt]

Also thanks, this was to get my feet wet with the repo and process, low benefit and low risk.

[smarter]

Thanks to you for porting this! And sorry for the long delay before the review, it slipped my mind.