uint256_ts are lower case by default, when converted to json (#31)

* Lower-case hex strings are the default for json output

* Remove debug printing

include/bigint.h

@@ -88,7 +88,7 @@ namespace boost
   {
     inline void to_json(nlohmann::json& j, const uint256_t& v)
     {
-      j = to_hex_str(v);
+      j = to_lower_hex_str(v);
     }
 
     inline void from_json(const nlohmann::json& j, uint256_t& v)

include/util.h

@@ -101,9 +101,6 @@ namespace evm
     // Start at index 2 to skip the "0x" prefix
     const auto h = keccak_256(s, 2);
 
-    std::cout << s << std::endl;
-    std::cout << to_hex_string(h) << std::endl;
-
     for (size_t i = 0; i < s.size() - 2; ++i)
     {
       auto& c = s[i + 2];