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/log/backtrace3.c

@@ -21,8 +21,8 @@
 #include "libc/assert.h"
 #include "libc/bits/weaken.h"
 #include "libc/calls/calls.h"
-#include "libc/conv/itoa.h"
 #include "libc/fmt/fmt.h"
+#include "libc/fmt/itoa.h"
 #include "libc/log/backtrace.internal.h"
 #include "libc/macros.h"
 #include "libc/nexgen32e/gc.internal.h"