Fold conv package into fmt

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

third_party/duktape/duk_config.h

@@ -44,8 +44,8 @@
 #include "libc/calls/termios.h"
 #include "libc/calls/typedef/sighandler_t.h"
 #include "libc/calls/weirdtypes.h"
-#include "libc/conv/conv.h"
 #include "libc/errno.h"
+#include "libc/fmt/conv.h"
 #include "libc/fmt/fmt.h"
 #include "libc/inttypes.h"
 #include "libc/limits.h"