Use once_cell::Lazy instead of std::sync::Once

[sdroege]

See summary. Simpler and less easy to get wrong.

[GuillaumeGomez]

I was sure we were already using it...

[sdroege]

git grep Once src :P

[GuillaumeGomez]

Your knowledge is too huge for me to handle. :p

[sdroege]

Requires re-exporting once_cell from glib though, it's used by a macro. So maybe something for once this is merged into std :)

[bilelmoussaoui]

This is only used in glib-macros, should we still port to once_cell even if we want to potentially get rid of it in the future?

[sdroege]

glib already re-exports once_cell or not?

[bilelmoussaoui]

Yes it does