ws2812: generate assembly instead of handwriting it

Previously, a new implementation had to be written for each processor
speed. This is tedious and error-prone, therefore I've rewritten all of
the assembly code using a tool that will calculate exactly how many NOP
instructions are required.

I've also switched from build tags to a switch statement over all
processor speeds.

All in all, this means that:
  - Chips with a supported CPU performance will automatically be
    supported (no build tags necessary).
  - Adding a new chip is as easy as adding the MHz count to the
    generator script and to the switch statement.

Right now this is only for the Cortex-M, but it's certainly feasible to
extend this to other architectures such as the AVR.

ws2812/ws2812_cortexm.go

@@ -0,0 +1,34 @@
+// +build cortexm
+
+package ws2812
+
+// This file implements the WS2812 protocol for various Cortex-M
+// microcontrollers. It is intended to work with various variants: M0, M0+, M3,
+// and M4. Because machine.CPUFrequency() is usually a constant, the value will
+// usually be constant-propagated and the switch below will be a direct
+// (inlinable function) - thus there is usually no code size penalty over build
+// tags per CPU speed.
+
+import (
+	"machine"
+)
+
+// Send a single byte using the WS2812 protocol.
+func (d Device) WriteByte(c byte) error {
+	switch machine.CPUFrequency() {
+	case 16_000_000: // 16MHz
+		d.writeByte16(c)
+		return nil
+	case 48_000_000: // 48MHz
+		d.writeByte48(c)
+		return nil
+	case 64_000_000: // 64MHz
+		d.writeByte64(c)
+		return nil
+	case 120_000_000: // 120MHz
+		d.writeByte120(c)
+		return nil
+	default:
+		return errUnknownClockSpeed
+	}
+}