Regex: fix invalid #inspect result against %r{\/} (crystal-lang#5841)

`p %r{\/}` shows `/\\//`. It is invalid regexp.

spec/std/regex_spec.cr

@@ -33,10 +33,12 @@ describe "Regex" do
 
   it "does inspect with slash" do
     %r(/).inspect.should eq("/\\//")
+    %r(\/).inspect.should eq("/\\//")
   end
 
   it "does to_s with slash" do
     %r(/).to_s.should eq("(?-imsx:\\/)")
+    %r(\/).to_s.should eq("(?-imsx:\\/)")
   end
 
   it "doesn't crash when PCRE tries to free some memory (#771)" do

src/regex.cr

@@ -519,12 +519,18 @@ class Regex
   end
 
   private def append_source(io)
-    source.each_char do |char|
-      if char == '/'
+    reader = Char::Reader.new(source)
+    while reader.has_next?
+      case char = reader.current_char
+      when '\\'
+        io << '\\'
+        io << reader.next_char
+      when '/'
         io << "\\/"
       else
         io << char
       end
+      reader.next_char
     end
   end