Fix ASTNode#to_s to use escape sequence correctly (crystal-lang#5842)

* Fix ASTNode#to_s to use escape sequence correctly

ASTNode#to_s against StringInterpolation, RegexLiteral, and Call of
backtick operator is buggy, it is missing to use escape sequence.

For example `pp "#{1}\0"` cannot be compiled by current compiler.

* Regex: make append_source as class method for RegexLiteral#to_s

* Use Regex.append_source directly

* Remove escape_regex and use Regex.append_source directly for all

crystal-lang#5842 (comment)

It is super tricky way.
But @straight-shoota said, so it is right way also.

spec/compiler/parser/to_s_spec.cr

@@ -125,4 +125,10 @@ describe "ASTNode#to_s" do
   expect_to_s %(if (1 + 2\n3)\n  4\nend)
   expect_to_s "%x(whoami)", "`whoami`"
   expect_to_s %(begin\n  ()\nend)
+  expect_to_s %q("\e\0\""), %q("\e\u0000\"")
+  expect_to_s %q("#{1}\0"), %q("#{1}\u0000")
+  expect_to_s %q(%r{\/\0}), %q(/\/\0/)
+  expect_to_s %q(%r{#{1}\/\0}), %q(/#{1}\/\0/)
+  expect_to_s %q(`\n\0`), %q(`\n\u0000`)
+  expect_to_s %q(`#{1}\n\0`), %q(`#{1}\n\u0000`)
 end

src/compiler/crystal/syntax/to_s.cr

@@ -102,7 +102,7 @@ module Crystal
 
     def visit(node : StringInterpolation)
       @str << '"'
-      visit_interpolation node, &.gsub('"', "\\\"")
+      visit_interpolation node, &.inspect_unquoted
       @str << '"'
       false
     end
@@ -499,9 +499,9 @@ module Crystal
       @str << '`'
       case exp
       when StringLiteral
-        @str << exp.value.inspect[1..-2]
+        @str << exp.value.inspect_unquoted.gsub('`', "\\`")
       when StringInterpolation
-        visit_interpolation exp, &.gsub('`', "\\`")
+        visit_interpolation exp, &.inspect_unquoted.gsub('`', "\\`")
       end
       @str << '`'
       false
@@ -968,9 +968,9 @@ module Crystal
       @str << '/'
       case exp = node.value
       when StringLiteral
-        @str << exp.value.gsub('/', "\\/")
+        Regex.append_source exp.value, @str
       when StringInterpolation
-        visit_interpolation exp, &.gsub('/', "\\/")
+        visit_interpolation(exp) { |s| Regex.append_source s, @str }
       end
       @str << '/'
       @str << 'i' if node.options.includes? Regex::Options::IGNORE_CASE

src/regex.cr

@@ -407,7 +407,7 @@ class Regex
   # ```
   def inspect(io : IO)
     io << '/'
-    append_source(io)
+    Regex.append_source(source, io)
     io << '/'
     io << 'i' if options.ignore_case?
     io << 'm' if options.multiline?
@@ -514,11 +514,12 @@ class Regex
     io << 'x' unless options.extended?
 
     io << ':'
-    append_source(io)
+    Regex.append_source(source, io)
     io << ')'
   end
 
-  private def append_source(io)
+  # :nodoc:
+  def self.append_source(source, io) : Nil
     reader = Char::Reader.new(source)
     while reader.has_next?
       case char = reader.current_char