fuzz: gitignore cargo-fuzz corpus/artifact files

If we want to keep seeds from these directories we should move them to the
qa-assets repo and make some effort to minimize them. It's easy to get 10s
of 000s of these files if you're just running the fuzzer without making an
effort to clean them up.

Author: apoelstra

.gitignore

@@ -4,6 +4,7 @@ Cargo.lock
 *~
 
 #fuzz
-fuzz/hfuzz_target
-fuzz/hfuzz_workspace
+fuzz/corpus
+fuzz/artifacts
+fuzz/*.log