Spinning multiple threads in this test causes a deadlock in
SGX where thread scheduling is not preemptive.

This commit refactors the initial implementation to fit into std and
makes some other changes:

- use MaybeUninit internally in SyncOnceCell
- correctly impl Drop for lazy::Once
- port Lazy::take from once_cell from: https://github.com/matklad/once_cell/pull/100Co-Authored-By: NPaul Dicker <pitdicker@users.noreply.github.com>