**inits** +containers

*O(n)*. Returns a sequence of all prefixes of this sequence, shortest first. For example,
> inits (fromList "abc") = fromList [fromList "", fromList "a", fromList "ab", fromList "abc"]
Evaluating the *i*th prefix takes *O(log(min(i, n-i)))*, but evaluating every prefix in the sequence takes *O(n)* due to sharing.