Test constexpr compatibility for all combinators

[simonask]

Currently, many things are marked constexpr that may or may not actually be usable in constexpr contexts due to the use of assertions. Whether assert() is usable in a constexpr context seems compiler-dependent.

Here is a solution for implementing assertions in constexpr contexts: https://akrzemi1.wordpress.com/2017/05/18/asserts-in-constexpr-functions/