Prefer binary realloc to self-managed buffers

Use erlang's allocator and simplify the code by writing into the
buffer we're going to give back anyway.

Also introduce a function to NUL-terminate iolist buffers, which can
avoid memory copying.

c_src/geef.h

@@ -29,4 +29,7 @@ typedef struct {
 
 extern geef_atoms atoms;
 
+/** NUL-terminate a binary */
+int geef_terminate_binary(ErlNifBinary *bin);
+
 #endif