age: Add Decryptor::new_buffered

This is significantly more efficient than `Decryptor::new` at parsing
headers, due to avoiding repeated short reads.

fuzz/Cargo.toml

@@ -34,3 +34,9 @@ path = "fuzz_targets/header.rs"
 [[bin]]
 name = "decrypt"
 path = "fuzz_targets/decrypt.rs"
+
+[[bin]]
+name = "decrypt_buffered"
+path = "fuzz_targets/decrypt_buffered.rs"
+test = false
+doc = false

fuzz/fuzz_targets/decrypt_buffered.rs

@@ -0,0 +1,18 @@
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+use std::iter;
+
+use age::Decryptor;
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(decryptor) = Decryptor::new_buffered(data) {
+        match decryptor {
+            Decryptor::Recipients(d) => {
+                let _ = d.decrypt(iter::empty());
+            }
+            // Don't pay the cost of scrypt while fuzzing.
+            Decryptor::Passphrase(_) => (),
+        }
+    }
+});