all: do not use machine.TWI_FREQ_* constants

These constants were always a bit weird:

  * they use an uncommon name (TWI instead of I2C)
  * they don't follow Go naming conventions (CamelCase)
  * the constants don't provide much value: their name is their value
    (100KHZ => 100e3).

Therefore, I propose we remove them and replace them with regular
numeric constants.

examples/hd44780i2c/main.go

@@ -14,7 +14,7 @@ func main() {
 	// use 3.3V (and may be damaged by 5V).
 
 	machine.I2C0.Configure(machine.I2CConfig{
-		Frequency: machine.TWI_FREQ_400KHZ,
+		Frequency: 400e3,
 	})
 
 	lcd := hd44780i2c.New(machine.I2C0, 0x27) // some modules have address 0x3F