ssd1306: avoid unnecessary heap allocations

[ysoldak]

This re-uses internal buffer to avoid unnecessary heap allocations in ssd1306 driver; switches away from the legacy I2C interface.

See similar: #766

Draft since not tested

[soypat]

It is seems to me Buser should have a command() method to avoid the big buffer juggling since commands always write a single byte.

type Buser interface {
    // ... other methods
    command(cmd byte) error
}
func (b *I2CBus) command(cmd byte) error {
	b.cmdbuf[0] = 0x00 // cmdbuf is only length 2 on I2CBus since SPI does some DC pin stuff. in place of 0x40/0x00 byte
	b.cmdbuf[1]= cmd 
	return b.wire.Tx(b.Address,b.cmdbuf[:],nil)
}

This will likely make it much more readable and easier to understand