refactor: Add consteval uint256(hex_str)

Complements uint256::FromHex() nicely in that it naturally does all error checking at compile time and so doesn't need to return an std::optional.

Will be used in the following 2 commits to replace many calls to uint256S(). uint256S() calls taking C-string literals are littered throughout the codebase and executed at runtime to perform parsing unless a given optimizer was surprisingly efficient. While this may not be a hot spot, it's better hygiene in C++20 to store the parsed data blob directly in the binary, without any parsing at runtime.

src/test/uint256_tests.cpp

@@ -399,4 +399,11 @@ BOOST_AUTO_TEST_CASE( check_ONE )
     BOOST_CHECK_EQUAL(one, uint256::ONE);
 }
 
+BOOST_AUTO_TEST_CASE(FromHex_vs_uint256)
+{
+    auto runtime_uint{uint256::FromHex("4A5E1E4BAAB89F3A32518A88C31BC87F618f76673e2cc77ab2127b7afdeda33b").value()};
+    constexpr uint256 consteval_uint{  "4a5e1e4baab89f3a32518a88c31bc87f618F76673E2CC77AB2127B7AFDEDA33B"};
+    BOOST_CHECK_EQUAL(consteval_uint, runtime_uint);
+}
+
 BOOST_AUTO_TEST_SUITE_END()