remove doctests that rely on non-default features

[williballenthin]

due to rust-lang/rust#67295, i understand that we cannot conditionally enable some features when running doctests. this results in failures like https://github.com/williballenthin/lancelot/runs/3000521975?check_suite_focus=true

i've hit this issues a few times in this project, and upon reflection, realize this was the reason i didn't implement #123 earlier.

since there is no proposed fix for rust-lang/rust#67295, then we should move doctests to tests, at least when they fail due to non-default features (like test or doctest).