Fix Visual Stdio build warning C26478

[Randalphwa]

Migrated from vector-of-bool/cmrc PR #27 by @czlitony

Fix Visual Stdio build warning C26478 Don't use std::move on constant variables. (es.56). which is caused by below codes:

    std::string filename() const && {
        return std::move(_fname);
    }

[Randalphwa]

(Migrated comment by @Pospelove on 6/21/2021 1:07:41 PM from original #27)

nice