Fold conv package into fmt

Both packages had nearly identical dependency requirements, so merging
them should help reduce the complexity of the build graph.

libc/calls/getrusage-nt.c

@@ -20,7 +20,7 @@
 #include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/struct/rusage.h"
-#include "libc/conv/conv.h"
+#include "libc/fmt/conv.h"
 #include "libc/nt/accounting.h"
 #include "libc/nt/runtime.h"
 #include "libc/nt/thread.h"