Simple implementation of a generic Compressed Sparse Row (CSR) matrix.

Features (an exhaustive list):
- The type of the values stored does not have any trait requirements.
- A separate builder type to populate the matrix with the desired values.
- Access all the populated entries in a matrix as a single iterator.
- Access the populated entries of a given row as a single iterator.

This crate was conceived for personal use, specifically for use in a WIP generic graphs crate.