Add redcarpet_ prefix for stack_* functions

The current redcarpet cannot be used together with ruby-prof because of occurring segmentation fault.  The reason why segv is occurred is that redcarpet and ruby-prof have functions with the same names, such as stack_push.  When you load ruby-prof before loading redcarpet, redcarpet internally uses ruby-prof's stack functions.

I think both libraries should change these generic names to library-specific names.