Matrix Keypad Driver

Provides an embedded Rust driver for a common numeric matrix keypad.

Example

```rust let rows = ( gpiob.pb15.intopullupinput(&mut gpiob.crh), gpioa.pa7.intopullupinput(&mut gpioa.crl), gpiob.pb6.intopullupinput(&mut gpiob.crl), gpioa.pa9.intopullupinput(&mut gpioa.crh), );

let cols = ( gpioa.pa8.intoopendrainoutput(&mut gpioa.crh), gpiob.pb5.intoopendrainoutput(&mut gpiob.crl), gpioc.pc7.intoopendrain_output(&mut gpioc.crl), );

let mut keypad = Keypad::new(rows, cols);

let key = keypad.read_char(&mut delay); if key != ' ' { ... } ```