negate after converting to unsigned to avoid UB on minimum signed value

[spoonincode]

Negating the minimum value of a signed integer is undefined behavior since the resulting value cannot be represented by the type (results in a signed overflow). However, casting to an unsigned and then negating that is defined behavior, and I believe the appropriate solution here, as both of those operations are well defined for all inputs.

[spoonincode]

This isn't consensus, though a change in console output would be.. undesirable. There is an existing test for std::numeric_limits<int128_t>::lowest() to give us some confidence that nothing has actually changed on supported platforms,

leap/unittests/test-contracts/test_api/test_print.cpp

Lines 35 to 48 in 6132e3b

	void test_print::test_printi128() {
	int128_t a(1);
	int128_t b(0);
	int128_t c(std::numeric_limits<int128_t>::lowest());
	int128_t d(-87654323456);
	printi128(&a);
	prints("\n");
	printi128(&b);
	prints("\n");
	printi128(&c);
	prints("\n");
	printi128(&d);
	prints("\n");
	}

leap/unittests/api_tests.cpp

Lines 2649 to 2660 in 6132e3b

	// test printi128
	auto tx6_trace = CALL_TEST_FUNCTION( *this, "test_print", "test_printi128", {} );
	auto tx6_act_cnsl = tx6_trace->action_traces.front().console;
	size_t start = 0;
	size_t end = tx6_act_cnsl.find('\n', start);
	BOOST_CHECK_EQUAL( tx6_act_cnsl.substr(start, end-start), U128Str(1) );
	start = end + 1; end = tx6_act_cnsl.find('\n', start);
	BOOST_CHECK_EQUAL( tx6_act_cnsl.substr(start, end-start), U128Str(0) );
	start = end + 1; end = tx6_act_cnsl.find('\n', start);
	BOOST_CHECK_EQUAL( tx6_act_cnsl.substr(start, end-start), "-" + U128Str(static_cast<unsigned __int128>(std::numeric_limits<__int128>::lowest())) );
	start = end + 1; end = tx6_act_cnsl.find('\n', start);
	BOOST_CHECK_EQUAL( tx6_act_cnsl.substr(start, end-start), "-" + U128Str(87654323456) );

[spoonincode]

I discovered two more cases of this behavior and decided to fix them as part of this PR in f68377f, I would like re-review when you have a chance @greg7mdp and @linh2931

WAST parsing is not consensus: it's only used in unit tests. In this particular case the WAST that tripped this error was from the test

leap/unittests/wasm_tests.cpp

Line 351 in fb9a0e7

BOOST_FIXTURE_TEST_CASE( f32_f64_conversion_tests, validating_tester ) try {

[greg7mdp]

Do we still need the I64 cast anymore?

[spoonincode]

it does seem completely redundant now, and everything still passes with it removed..

[greg7mdp]

Seems redundant to me as well. Probably better to remove it.

[spoonincode]

I was a little hesitant because this case is a little unique due to the u64->u32 truncation and wasn't sure if -UnsignedInt(u64) vs UnsignedInt(-u64) mattered.., but it seems equivalent.