ReprCString should have drop, like CVec

[kenfred]

If ReprCString is analogous to Rust's String and owns its own memory, it needs a drop method just like CVec. As it is currently, passing ReprCString to the C side will leak because it has no way to free the string.

[h33p]

This is true, thanks for reporting! Generally it's a bit tricky, because it's meant to kind of mimic regular C char * strings, so I'm not set whether it's a good idea to add a drop with each string, or make sure each string is allocated with malloc.