miniscript: Using Func and Expr when parsing keys, hashes, and locktimes

Since pk(), pk_k(), pkh(), pk_h(), sha256(), ripemd160(), hash256(),
hash160(), after(), and older() all are single argument expressions that
are parsed immediately, we can use the Expr and Func parsing functions
to determine what the arguments of these expressions are, rather than
searching for the next closing parentheses.

This fixes an issue when pk(), pk_k(), pkh(), and pk_h() include a
musig() expression as Expr properly handles nested expressions.

src/test/fuzz/miniscript.cpp

@@ -154,10 +154,9 @@ struct ParserContext {
         return {h.begin(), h.end()};
     }
 
-    template<typename I>
-    std::optional<Key> FromString(I first, I last) const {
-        if (last - first != 2) return {};
-        auto idx = ParseHex(std::string(first, last));
+    std::optional<Key> FromString(std::span<const char>& in) const {
+        if (in.size() != 2) return {};
+        auto idx = ParseHex(std::string(in.begin(), in.end()));
         if (idx.size() != 1) return {};
         return TEST_DATA.dummy_keys[idx[0]];
     }