unsafeInterleaveIO is used instead of unsafeDupableInterleaveIO because
a mk_supply thunk that is simultaneously entered by two threads should
evaluate to the same UniqSupply.

The UniqSupply counter is now incremented atomically using the RTS's
atomic_inc().

To mitigate the extra overhead of unsafeInterleaveIO in the
single-threaded compiler, noDuplicate# is changed to exit early when
n_capabilities == 1.