Remove 2 restrictions from `CONTRIBUTING.md`; close #5159

* Allow mixed declarations, especially loop variables
* Allow C++ style commends `//`

CONTRIBUTING.md

@@ -33,11 +33,6 @@ mruby should be highly portable to other systems and compilers. For this it is
 recommended to keep your code as close as possible to the C99 standard
 (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf).
 
-Although we target C99, we've heard some compilers in the embedded environment
-still requires declarations of local variables to be at the beginning of a
-scope. Until we confirm the situation has changed, we use the old-style
-variable declaration.
-
 Visual C++ is also an important target for mruby (supported version is 2013 or
 later). For this reason features that are not supported by Visual C++ may not
 be used (e.g. `%z` of `strftime()`).
@@ -50,12 +45,6 @@ The dependencies to libraries should be kept to an absolute minimum. This
 increases the portability but makes it also easier to cut away parts of mruby
 on-demand.
 
-#### Don't use C++ style comments
-
-    /* This is the preferred comment style */
-
-Use C++ style comments only for temporary comment e.g. commenting out some code lines.
-
 #### Insert a break after the function return value:
 
     int