message: Convert unreachable exception into assert
diff: #include "parser.hpp"
  #include "stack.hpp"
  +#include <cassert>
  namespace fizzy
  {
  @@ -393,8 +394,7 @@ parser_result<Code> parse_expr(const uint8_t* pos, const uint8_t* end)
  }
  code.instructions.emplace_back(instr);
  }
  - if (!label_positions.empty())
  - throw parser_error{"code is not balanced (missing end statements)"};
  + assert(label_positions.empty());
  return {code, pos};
  }
  } // namespace fizzy
  