Fixing additional signedness errors after adding -Wsign-conversion flag

Co-authored-by: Kamil Śliwak <kamil.sliwak@codepoets.it>

libsmtutil/CVC4Interface.cpp

@@ -191,7 +191,7 @@ CVC4::Expr CVC4Interface::toCVC4Expr(Expression const& _expr)
 			return m_context.mkExpr(CVC4::kind::BITVECTOR_AND, arguments[0], arguments[1]);
 		else if (n == "int2bv")
 		{
-			size_t size = std::stoi(_expr.arguments[1].name);
+			size_t size = std::stoul(_expr.arguments[1].name);
 			auto i2bvOp = m_context.mkConst(CVC4::IntToBitVector(size));
 			// CVC4 treats all BVs as unsigned, so we need to manually apply 2's complement if needed.
 			return m_context.mkExpr(

libsmtutil/SMTLib2Interface.cpp

@@ -139,7 +139,7 @@ string SMTLib2Interface::toSExpr(Expression const& _expr)
 	std::string sexpr = "(";
 	if (_expr.name == "int2bv")
 	{
-		size_t size = std::stoi(_expr.arguments[1].name);
+		size_t size = std::stoul(_expr.arguments[1].name);
 		auto arg = toSExpr(_expr.arguments.front());
 		auto int2bv = "(_ int2bv " + to_string(size) + ")";
 		// Some solvers treat all BVs as unsigned, so we need to manually apply 2's complement if needed.

libsmtutil/Z3Interface.cpp

@@ -184,7 +184,7 @@ z3::expr Z3Interface::toZ3Expr(Expression const& _expr)
 			return arguments[0] & arguments[1];
 		else if (n == "int2bv")
 		{
-			size_t size = std::stoi(_expr.arguments[1].name);
+			size_t size = std::stoul(_expr.arguments[1].name);
 			return z3::int2bv(size, arguments[0]);
 		}
 		else if (n == "bv2int")