[parser] adding chunked shuffle as a C function (writes each line to one of n random files, runs shuf on each file and concatenates the result). Adding a version which allows specifying a specific chunk size, and using a 2GB limit for address parser training. Allowing gshuf again for Mac as it seems the only problem there was not having enough memory when testing on a Mac laptop. The new limited-memory version should be fast enough.

src/shuffle.h

@@ -5,5 +5,7 @@
 #include <stdbool.h>
 
 bool shuffle_file(char *filename);
+bool shuffle_file_chunked(char *filename, size_t parts);
+bool shuffle_file_chunked_size(char *filename, size_t chunk_size);
 
 #endif