Add parseUInt utility function

There is an increasing number of places where Catch2 wants to parse
strings into numbers, but being stuck in C++14 world, we do not
have good stdlib facilities to do this (`strtoul` and `stoul`
are both bad).

src/catch2/internal/catch_parse_numbers.cpp

@@ -0,0 +1,49 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#include <catch2/internal/catch_parse_numbers.hpp>
+
+#include <catch2/internal/catch_compiler_capabilities.hpp>
+#include <catch2/internal/catch_string_manip.hpp>
+
+#include <limits>
+
+namespace Catch {
+
+    Optional<unsigned int> parseUInt(std::string const& input, int base) {
+        auto trimmed = trim( input );
+        // std::stoull is annoying and accepts numbers starting with '-',
+        // it just negates them into unsigned int
+        if ( trimmed.empty() || trimmed[0] == '-' ) {
+            return {};
+        }
+
+        CATCH_TRY {
+            size_t pos = 0;
+            const auto ret = std::stoull( trimmed, &pos, base );
+
+            // We did not consume the whole input, so there is an issue
+            // This can be bunch of different stuff, like multiple numbers
+            // in the input, or invalid digits/characters and so on. Either
+            // way, we do not want to return the partially parsed result.
+            if ( pos != trimmed.size() ) {
+                return {};
+            }
+            // Too large
+            if ( ret > std::numeric_limits<unsigned int>::max() ) {
+                return {};
+            }
+            return static_cast<unsigned int>(ret);
+        } CATCH_CATCH_ANON( std::exception const& ) {
+            // There was a larger issue with the input, e.g. the parsed
+            // number would be too large to fit within ull.
+            return {};
+        }
+    }
+
+} // namespace Catch