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/cp.c

@@ -9,8 +9,8 @@
 #endif
 #include "libc/calls/calls.h"
 #include "libc/calls/hefty/copyfile.h"
-#include "libc/conv/conv.h"
 #include "libc/errno.h"
+#include "libc/fmt/conv.h"
 #include "libc/fmt/fmt.h"
 #include "libc/runtime/gc.h"
 #include "libc/runtime/runtime.h"