util: Introduce ToIntegral<T>(const std::string&) for locale independent parsing using std::from_chars(…) (C++17)

util: Avoid locale dependent functions strtol/strtoll/strtoul/strtoull in ParseInt32/ParseInt64/ParseUInt32/ParseUInt64

fuzz: Assert equivalence between new and old Parse{Int,Uint}{8,32,64} functions

test: Add unit tests for ToIntegral<T>(const std::string&)

src/util/strencodings.h

@@ -12,8 +12,10 @@
 #include <attributes.h>
 #include <span.h>
 
+#include <charconv>
 #include <cstdint>
 #include <iterator>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -94,6 +96,24 @@ constexpr inline bool IsSpace(char c) noexcept {
     return c == ' ' || c == '\f' || c == '\n' || c == '\r' || c == '\t' || c == '\v';
 }
 
+/**
+ * Convert string to integral type T.
+ *
+ * @returns std::nullopt if the entire string could not be parsed, or if the
+ *   parsed value is not in the range representable by the type T.
+ */
+template <typename T>
+std::optional<T> ToIntegral(const std::string& str)
+{
+    static_assert(std::is_integral<T>::value);
+    T result;
+    const auto [first_nonmatching, error_condition] = std::from_chars(str.data(), str.data() + str.size(), result);
+    if (first_nonmatching != str.data() + str.size() || error_condition != std::errc{}) {
+        return std::nullopt;
+    }
+    return {result};
+}
+
 /**
  * Convert string to signed 32-bit integer with strict parse error feedback.
  * @returns true if the entire string could be parsed as valid integer,