util: Add ParseInt64 and ParseDouble functions

Strict parsing functions for other numeric types.

- ParseInt64 analogous to ParseInt32, but for 64-bit values.
- ParseDouble for doubles.
- Make all three Parse* functions more strict (e.g. reject whitespace on
  the inside)

Also add tests.

src/utilstrencodings.h

@@ -49,6 +49,20 @@ int atoi(const std::string& str);
  */
 bool ParseInt32(const std::string& str, int32_t *out);
 
+/**
+ * Convert string to signed 64-bit integer with strict parse error feedback.
+ * @returns true if the entire string could be parsed as valid integer,
+ *   false if not the entire string could be parsed or when overflow or underflow occurred.
+ */
+bool ParseInt64(const std::string& str, int64_t *out);
+
+/**
+ * Convert string to double with strict parse error feedback.
+ * @returns true if the entire string could be parsed as valid double,
+ *   false if not the entire string could be parsed or when overflow or underflow occurred.
+ */
+bool ParseDouble(const std::string& str, double *out);
+
 template<typename T>
 std::string HexStr(const T itbegin, const T itend, bool fSpaces=false)
 {