Remove `$/` from mruby implementation.

1. `$/` and other Perl-ish global variables are not defined in ISO.
2. The current Ruby policy do not encourage those variables.
3. Those variables has global effect and can cause troubles.

mrbgems/mruby-io/mrblib/io.rb

@@ -226,12 +226,12 @@ class IO
     end
   end
 
-  def readline(arg = $/, limit = nil)
+  def readline(arg = "\n", limit = nil)
     case arg
     when String
       rs = arg
     when Fixnum
-      rs = $/
+      rs = "\n"
       limit = arg
     else
       raise ArgumentError
@@ -242,7 +242,7 @@ class IO
     end
 
     if rs == ""
-      rs = $/ + $/
+      rs = "\n\n"
     end
 
     array = []

mrbgems/mruby-io/src/io.c

@@ -1328,7 +1328,4 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "closed?",    mrb_io_closed,     MRB_ARGS_NONE());   /* 15.2.20.5.2 */
   mrb_define_method(mrb, io, "pid",        mrb_io_pid,        MRB_ARGS_NONE());   /* 15.2.20.5.2 */
   mrb_define_method(mrb, io, "fileno",     mrb_io_fileno,     MRB_ARGS_NONE());
-
-
-  mrb_gv_set(mrb, mrb_intern_cstr(mrb, "$/"), mrb_str_new_cstr(mrb, "\n"));
 }

src/string.c

@@ -1529,9 +1529,8 @@ mrb_str_chomp_bang(mrb_state *mrb, mrb_value str)
  *     str.chomp(separator="\n")   => new_str
  *
  *  Returns a new <code>String</code> with the given record separator removed
- *  from the end of <i>str</i> (if present). If <code>$/</code> has not been
- *  changed from the default Ruby record separator, then <code>chomp</code> also
- *  removes carriage return characters (that is it will remove <code>\n</code>,
+ *  from the end of <i>str</i> (if present). <code>chomp</code> also removes
+ *  carriage return characters (that is it will remove <code>\n</code>,
  *  <code>\r</code>, and <code>\r\n</code>).
  *
  *     "hello".chomp            #=> "hello"