use suffix array for every prefix when deciding the lower bound of a ts

[sebschmi]

if the min length of a ts is n, but the longest prefix match is only n/2, then give costs assuming that the first n/2 characters are correct and then there is one error

[jeeeesper]

rust-bio (#13) has algorithms to create suffix arrays.

[jeeeesper]

Relates to #5 and #65