Update fuzz target to afl 0.13

Author: dtolnay

fuzz/Cargo.toml

@@ -9,7 +9,7 @@ publish = false
 cargo-fuzz = true
 
 [dependencies]
-afl = { version = "0.12", optional = true }
+afl = { version = "0.13", optional = true }
 honggfuzz = { version = "0.5", optional = true }
 libfuzzer-sys = { version = "0.4", optional = true }
 proc-macro2 = { path = "..", default-features = false }