The previous implementation, itself based on an earlier version of
`Data.List.inits`, inherited the flaw that accessing the i-th element took
quadratic time O(i²). This now takes linear time O(i) as expected.

The current version of `Data.List.inits` uses a banker's queue to
obtain good performance when generating very long lists.
For lazy text, consisting of a few big chunks, that benefit seems
negligible. So I chose a simpler implementation.