fuzz: rename and improve the Miniscript Script roundtrip target

Parse also key hashes using the Key type. Make this target the first of
the 4 Miniscript fuzz targets in a single `miniscript` file.

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>

src/test/fuzz/miniscript.cpp

@@ -0,0 +1,70 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <core_io.h>
+#include <hash.h>
+#include <key.h>
+#include <script/miniscript.h>
+#include <script/script.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+#include <util/strencodings.h>
+
+namespace {
+
+//! Context that implements naive conversion from/to script only, for roundtrip testing.
+struct ScriptParserContext {
+    //! For Script roundtrip we never need the key from a key hash.
+    struct Key {
+        bool is_hash;
+        std::vector<unsigned char> data;
+    };
+
+    const std::vector<unsigned char>& ToPKBytes(const Key& key) const
+    {
+        assert(!key.is_hash);
+        return key.data;
+    }
+
+    const std::vector<unsigned char> ToPKHBytes(const Key& key) const
+    {
+        if (key.is_hash) return key.data;
+        const auto h = Hash160(key.data);
+        return {h.begin(), h.end()};
+    }
+
+    template<typename I>
+    std::optional<Key> FromPKBytes(I first, I last) const
+    {
+        Key key;
+        key.data.assign(first, last);
+        key.is_hash = false;
+        return key;
+    }
+
+    template<typename I>
+    std::optional<Key> FromPKHBytes(I first, I last) const
+    {
+        Key key;
+        key.data.assign(first, last);
+        key.is_hash = true;
+        return key;
+    }
+} SCRIPT_PARSER_CONTEXT;
+
+}
+
+/* Fuzz tests that test parsing from a script, and roundtripping via script. */
+FUZZ_TARGET(miniscript_script)
+{
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+    const std::optional<CScript> script = ConsumeDeserializable<CScript>(fuzzed_data_provider);
+    if (!script) return;
+
+    const auto ms = miniscript::FromScript(*script, SCRIPT_PARSER_CONTEXT);
+    if (!ms) return;
+
+    assert(ms->ToScript(SCRIPT_PARSER_CONTEXT) == *script);
+}