Provide constructor for bit_bang_i2c that accepts strong_ptr pins

```C++
class bit_bang_i2c : public i2c
{
public:
  struct pins
  {
    output_pin* sda;
    output_pin* scl;
  };
```

This is what we have currently, what would be great is if we had some constructor that took pins that are strong_ptrs instead. 

```C++

  struct safe_pins
  {
    hal::v5::strong_ptr<output_pin> sda;
    hal::v5::strong_ptr<output_pin> scl;
  };
```