is_stream_insertable_v and reserved identifier fix

[arturbac]

Problem:

• constraints on eq #615

Solution:

• is_stream_insertable_v and reserved identifier fix

Checked locally with gcc and clang, added few tests
Strange feeling to write c++17 code after many month of only c++20 and up .. hard to examine if the type trait have sense .. i hate old enable_if_t , long live concepts ;-)

[krzysztof-jusiak]

Thanks @arturbac

[arturbac]

Thanks @krzysztof-jusiak for great lib.