Consider to implement left shift operator by multiplication in C++

[mikir]

The left shift of a signed integer is officially undefined behavior in C++17 and earlier: https://en.cppreference.com/w/cpp/language/operator_arithmetic#Bitwise_shift_operators

In C++20, signed integers are officially two's-complement, so left-shift becomes well defined: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0907r4.html.

Consider to fix it by generating the following code instead of a << b:

a * (1 << b);

This is issue is connected to the Zserio issue #671.