Remove ToNucleotideLike

[swooster]

The current version of NucleotideIter works with iterators of Nucleotide, &Nucleotide, NucleotideAmbiguous, &NucleotideAmbiguous and impl NucleotideLike, but not &impl NucleotideLike. For example the following code currently doesn't compile:

use quickdna::{NucleotideIter, NucleotideLike};

fn do_stuff(dna: &[impl NucleotideLike]) {
    dna.iter().reverse_complement();
}

This PR removes the kludgey infectious ToNucleotideLike trait I came up with and instead of relies on how type resolution of methods is a bit smarter than traits.

[swooster]

If this is too large to review, let me know and I may be able to split it into some smaller PRs. (no promises until I try it though)

[franssl]

Looks good!