Codegen: don't use init flag for string constants

[asterite]

Also includes a small refacor.

For example if you have code like this:

puts FOO

FOO = "hello"

Then before this PR when reading FOO we would check whether it was initialized or not. This is not needed before a string literal doesn't need a special initialization, it's just data.

Like with #9801, it's not common to find code like the above but it is common that this happens because of order of require and the way the compiler works.

Benchmark (Int#to_s uses some constants that are string literals):

require "benchmark"

null = File.open(File::NULL, "w")

Benchmark.ips do |x|
  x.report("Int#to_s") do
    123456789.to_s(null)
  end
end

Before:

Int#to_s  89.91M ( 11.12ns) (± 4.98%)  0.0B/op

After:

Int#to_s  97.35M ( 10.27ns) (± 5.22%)  0.0B/op  fastest

[asterite]

Oh, it's failing with the Crystal constants which aren't in the program's AST. I'll fix that later.

[bcardiff]

@asterite can you rebase on master?

[asterite]

Sure! Just note that you can also do it because the branch is in this repo 😉

[asterite]

Done!