sets the required standard to C++20

[cjdb]

CMake was compiling with -std=gnu++1z by default, which apparently causes some strange functionality (see https://godbolt.org/z/g6duRE). This commit sets the minimum standard to the closest standard to either C++2a or C++20 and disables compiler extensions.

[CaseyCarter]

CMake Error in examples/CMakeLists.txt:
  The compiler feature "cxx_std_20" is not known to CXX compiler

  "GNU"

  version 7.3.0.

While we don't precisely need any particular 20 features given support for TS concepts, I'm inclined to cut the cord anyway and drop support for GCC7. "Most recent two compiler releases" is reasonable support for a research tool like cmcstl2.

Opinions?

[cjdb]

While we don't precisely need any particular 20 features given support for TS concepts, I'm inclined to cut the cord anyway and drop support for GCC7. "Most recent two compiler releases" is reasonable support for a research tool like cmcstl2.

Opinions?

I'm in favour, but I think we'll need to fix #311 first.

[CaseyCarter]

I'm in favour, but I think we'll need to fix #311 first.

Actually supporting GCC9 is obviously a prerequisite for GCC9 support; there's no need to make any changes (except to the CI scripts) to drop GCC7 support.