Fold conv package into fmt

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

test/libc/test.mk

@@ -6,7 +6,6 @@ o/$(MODE)/test/libc:					\
 		o/$(MODE)/test/libc/alg			\
 		o/$(MODE)/test/libc/bits		\
 		o/$(MODE)/test/libc/calls		\
-		o/$(MODE)/test/libc/conv		\
 		o/$(MODE)/test/libc/crypto		\
 		o/$(MODE)/test/libc/dns			\
 		o/$(MODE)/test/libc/fmt			\