Backporting the library to C++11

[gennaroprota]

Vinnie expressed a desire to make this library C++11. Here's a breakdown of the post-C++11 core and library features that are currently used:

• Concepts: These can be replaced with SFINAE-based constructs. There's just a handful of these.

• if constexpr with branches that would not compile simultaneously: Can be replaced with tag dispatching, std::enable_if or template specializations. There are about 40 occurrences of these.

• Nested namespace definitions: Easily replaceable. There are just 18 of these.

• operator<=>: Replaceable by separate comparison operators. There are about 10 of these, which would produce about 60 comparison operators.

• std::format() and std::println(): Used pervasively (about 50 occurrences). Can be replaced with string concatenation/iostreams. Some types have their formatters, so we should define stream inserters for them.

• std::string_view: Also used pervasively (460+ occurrences). Can be replaced with std::string or char const *, as appropriate.

[vinniefalco]

Can be replaced with std::string or char const *

Or boost::core::detail::string_view