Switched C-API to default to safe RSTRING.

If a C-ext modifies the RSTRING buffer, add the following:

  #define RSTRING_MODIFIED 1

to the appropriate header file or source file.

Since many performance issues related to C-exts are actually the RSTRING
caching code, we'd prefer to have C-exts break up front and fix them
than leave harder-to-find performance issues that mistakenly attribute
poor performance to Rubinius itself.