util: move spanparsing.h to script/parsing.h

Move miniscript / descriptor script parsing functions out of util library so
they are not a dependency of the kernel.

There are no changes to code or behavior.

src/script/parsing.cpp

@@ -0,0 +1,52 @@
+// Copyright (c) 2018-2022 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 <script/parsing.h>
+
+#include <span.h>
+
+#include <algorithm>
+#include <cstddef>
+#include <string>
+
+namespace script {
+
+bool Const(const std::string& str, Span<const char>& sp)
+{
+    if ((size_t)sp.size() >= str.size() && std::equal(str.begin(), str.end(), sp.begin())) {
+        sp = sp.subspan(str.size());
+        return true;
+    }
+    return false;
+}
+
+bool Func(const std::string& str, Span<const char>& sp)
+{
+    if ((size_t)sp.size() >= str.size() + 2 && sp[str.size()] == '(' && sp[sp.size() - 1] == ')' && std::equal(str.begin(), str.end(), sp.begin())) {
+        sp = sp.subspan(str.size() + 1, sp.size() - str.size() - 2);
+        return true;
+    }
+    return false;
+}
+
+Span<const char> Expr(Span<const char>& sp)
+{
+    int level = 0;
+    auto it = sp.begin();
+    while (it != sp.end()) {
+        if (*it == '(' || *it == '{') {
+            ++level;
+        } else if (level && (*it == ')' || *it == '}')) {
+            --level;
+        } else if (level == 0 && (*it == ')' || *it == '}' || *it == ',')) {
+            break;
+        }
+        ++it;
+    }
+    Span<const char> ret = sp.first(it - sp.begin());
+    sp = sp.subspan(it - sp.begin());
+    return ret;
+}
+
+} // namespace script