benchmark the `linalg` module

this MR

• adds criterion as a dependency
• creates a linalg.rs benchmark file
• makes the following function public
  • 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