Rename `matrix` module to `dense`

src/lib.rs

 //! Fast position-weight matrices using sequence striping and SIMD.
 
 mod abc;
-mod matrix;
+mod dense;
 mod seq;

src/seq.rs

 use super::abc::Alphabet;
 use super::abc::InvalidSymbol;
-use super::matrix::DenseMatrix;
+use super::dense::DenseMatrix;
 
 #[derive(Clone, Debug)]
 pub struct EncodedSequence<A: Alphabet> {