Fold conv package into fmt

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

examples/unbourne.c

@@ -120,9 +120,9 @@
 #include "libc/calls/struct/stat.h"
 #include "libc/calls/struct/tms.h"
 #include "libc/calls/termios.h"
-#include "libc/conv/conv.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
+#include "libc/fmt/conv.h"
 #include "libc/fmt/fmt.h"
 #include "libc/limits.h"
 #include "libc/log/log.h"