Add licenses in all source files

[OTL]

[taiki-e]

As far as I know, per-file license notifications are not always necessary. Actually, rust-lang/rust removed it from all files: rust-lang/rust#57108

However, it is preferable that all published crates have license file. This is required when redistributing crates. See mmastrac/rust-ctor#109 for more.

[OTL]

I see, thanks!
I close this issue.