Make use of C++20 features

[eigenraven]

A few useful features:

• std::span instead of ptr+len pairs and some const vector&/const string& references as it can accept any of these, and avoid copying memory
• std::jthread stoppable threads - might be good to express threads that must join after they get destructed, especially in testsuites
• ranges can express many simple algorithms like find-and-remove, etc. more compactly and readably
• designated initializers for simple structs, this lets them be constructed like {.field1 = 7, .field2 = {.x=1, y=2}} which could be useful for some small structs that get passed around

Refer to https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B20_features for compiler support, we now use clang-13 with GNU libstdc++11

[Shillaker]

I would also like to investigate using modules to divide along the existing library lines in the CMake project structure. This may improve build performance and tidy up the code, but it could also just add needless verbosity and introduce some compatibility issues with non-Clang compilers.

[Shillaker]

We can also replace our Latch and Barrier in Faabric using std::latch and std::barrier.