this MR
- adds `criterion` as a dependency
- creates a `linalg.rs` benchmark file
- makes the following function `pub`lic
  - `Matrix::transpose`
  - `Matrix::invert`
  - `Matrix::mul`
- creates a new `benches/` directory containing
  - a README with commands
  - a `plot.py` file to plot results
  - a `linalg.rs` file with the benchmarks

## example results
![Figure_1](/uploads/f352a6f411662361fa9ca381710271d5/Figure_1.png)