general: Use std's `OnceCell` where possible
Rust's standard library now provides a drop-in replacement
for OnceCell
. It requires no API changes from once_cell
's
version.
This commit replaces the version from once_cell
with the
new std
struct where possible. We still need to use
once_cell
for properties until
https://github.com/gtk-rs/gtk-rs-core/pull/1113
is merged. We also still need it for Lazy until that lands.