Added i2c slave support

- Callbacks can be called after each read/write cycle or per byte
 - Each I2C module can have different callbacks
 - General call support also working
 - Supports master and slave at same time. Also works with multimaster

Usage:

i2c_msg msg;
char buffer[255];

main()
{
    i2c_slave_enable(I2C1, I2C_FAST_MODE | I2C_SLAVE_DUAL_ADDRESS | I2C_SLAVE_GENERAL_CALL | I2C_SLAVE_USE_RX_BUFFER); // init slave mode. Enables master too
    i2c_slave_attach_recv_handler(I2C1, pmsg, funcrx); // attach  receive handler
    i2c_slave_attach_transmit_handler(I2C1, pmsg, functx);   // attach transmit handler
    i2c_slave_set_own_address(I2C1, 0x10);    // set addresss 1
    i2c_slave_set_own_address2(I2C1, 0x20);   // set addresss 2
}

void funcrx(i2c_msg *msg)
{
    printf("length is %d.\n", msg->length);
    char return_data = msg0>data[0];
}

void functx(i2c_msg *dev)
{
    msg->data[0] = 0x01;
    msg->data[1] = 0x02;
    msg->data[2] = 0x03;
    msg->data[3] = 0x04;
    msg->data[4] = 0x05;
    msg->length = 5;
}

All code derived from datasheets and libmaple.
Signed-off-by:- Barry Carter <barry.carter@gmail.com>