Do not use globals for regex

[asterite]

This is mainly a compiler refactor.

When you use a regex literal:

/hello/

what the compiler did, before this PR, is something like this:

if (temp = $global)
  temp
else
  $global = Regex.new("hello")
end

YES, it was using global variables, which we removed from the language a long time ago. But the code around them was still kept because it was used for regex.

This is all fine, except that if, say, someone wants to write an interpreter for crystal ( 😉 ), they will have to implement global variables just for this.

This PR changes that logic to use a constant instead. So now it's more like this:

GLOBAL

Suspicious... where is GLOBAL declared? It's done internally as part of expanding the regex literal, there's no visible assignment to it. But it works!

An extra benefit is that the old check wasn't fiber/thread safe. Reading a constant is fiber/thread safe, so this isn't just a refactor to make implementing the interpreter easier to do.

With that, all the logic related to global variables is removed from the compiler. There was an AST node named Global which was still used for the special variables $~ and $?: this PR renames this node to SpecialVar.

[HertzDevil]

Does renaming an AST node type name not constitute a breaking change to the macro language?

[asterite]

Does renaming an AST node type name not constitute a breaking change to the macro language?

Oh, I guess so... Hmm... I'll just undo the last commit then.

[straight-shoota]

A PR with lots of code removals that still brings actual improvements is really lovely.

[asterite]

Do we know why CI fails? It seems to be failing in a lot of PRs...

[straight-shoota]

I don't know what's wrong with aarch64 specs. But it doesn't seem to be related to this patch.

[HertzDevil]

Before and after, prepared using a local instance of Compiler Explorer: https://gist.github.com/HertzDevil/182951aa615da448752729d71a45705d

[asterite]

Do you think it's better or worse? I guess worse because of the atomic initialization, right?