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/bits/abs.c

@@ -17,7 +17,9 @@
 │ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
 │ 02110-1301 USA                                                               │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/conv/conv.h"
+#include "libc/fmt/conv.h"
 #include "libc/macros.h"
 
-int(abs)(int x) { return ABS(x); }
+int(abs)(int x) {
+  return ABS(x);
+}