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/stb/stb_vorbis.c

@@ -36,7 +36,7 @@
 #include "libc/assert.h"
 #include "libc/bits/bits.h"
 #include "libc/calls/calls.h"
-#include "libc/conv/conv.h"
+#include "libc/fmt/conv.h"
 #include "libc/limits.h"
 #include "libc/math.h"
 #include "libc/mem/alloca.h"