host: migrate functions to subpackage cpu. (#63)

Otherwise this would cause some drivers to import host, which would import
drivers that the user may not desire.

This commit fixes this import loop.

devices/tm1637/tm1637.go

@@ -15,7 +15,7 @@ import (
 	"time"
 
 	"github.com/google/periph/conn/gpio"
-	"github.com/google/periph/host"
+	"github.com/google/periph/host/cpu"
 )
 
 // Clock converts time to a slice of bytes as segments.
@@ -184,5 +184,5 @@ func (d *Dev) writeByte(b byte) (bool, error) {
 
 // sleep does a busy loop to act as fast as possible.
 func (d *Dev) sleepHalfCycle() {
-	host.Nanospin(clockHalfCycle)
+	cpu.Nanospin(clockHalfCycle)
 }

experimental/devices/bitbang/i2c.go

@@ -17,7 +17,7 @@ import (
 
 	"github.com/google/periph/conn/gpio"
 	"github.com/google/periph/conn/i2c"
-	"github.com/google/periph/host"
+	"github.com/google/periph/host/cpu"
 )
 
 // Use SkipAddr to skip the address from being sent.
@@ -251,7 +251,7 @@ func (i *I2C) readByte() (byte, error) {
 
 // sleep does a busy loop to act as fast as possible.
 func (i *I2C) sleepHalfCycle() {
-	host.Nanospin(i.halfCycle)
+	cpu.Nanospin(i.halfCycle)
 }
 
 var _ i2c.Bus = &I2C{}

experimental/devices/bitbang/spi.go

@@ -19,7 +19,7 @@ import (
 
 	"github.com/google/periph/conn/gpio"
 	"github.com/google/periph/conn/spi"
-	"github.com/google/periph/host"
+	"github.com/google/periph/host/cpu"
 )
 
 // SPI represents a SPI master implemented as bit-banging on 3 or 4 GPIO pins.
@@ -164,7 +164,7 @@ func NewSPI(clk, mosi gpio.PinOut, miso gpio.PinIn, cs gpio.PinOut, speedHz int6
 
 // sleep does a busy loop to act as fast as possible.
 func (s *SPI) sleepHalfCycle() {
-	host.Nanospin(s.halfCycle)
+	cpu.Nanospin(s.halfCycle)
 }
 
 var _ spi.Conn = &SPI{}