Switch from deprecated double-checked-cell to once_cell

[chriskrycho]

trafficstars

double-checked-cell is deprecated and recommends migrating to once_cell. It's not urgent, but worth putting on our radar.

[charlespierce]

Oh nice! IIRC, at the time I looked at this once_cell didn't have the get_or_try_init method, which made it very awkward to initialize the cell with a fallible operation. That ergonomic API is ultimately what led me to double-checked-cell, but it looks like that's since been incorporated, so agreed we should look to switch over.