Fix from_ruby<char const*> to return Ruby String handle directly

[ryannevell]

#97

Rice::String(x).str() would create a temporary std::string on the stack, and returning a pointer to its data could result in memory corruption when newly malloc'd memory was changed. This change removes the need for that temporary copy, and instead returns a pointer to the underlying Ruby string's memory, which will not be freed until that Ruby object is off of the C/C++ stack and is unreachable by Ruby code.

[jasonroelofs]

Looks like the test suite is good but failing on Ruby 2.4. I'll get that fixed in another change set.

The current iteration of this method came from 7efbd5c which looking at the change to const char * that may not have been the right thing to change.

I can't think of any reason not to apply this change. Chances are people won't be trying to grab binary data in a const char * from a Ruby String (no knowledge of length), but should instead use a std::string. I'll do some quick testing locally and get this in.

Thanks!