Make sized strings for NFA

[elefthei]

We should be able to use [char;1] instead of String everywhere and maybe
const EPSILON: [char; 1] = [ '\0' ], this would allow us to get the Copy trait for strings which should lead to less/no heap operations. For double stride, some size polymorphism is needed so that DFA<[char;n]> -> DFA<[char;2*n]>