Release 0.3.0

Version 0.3.0 has added support for macOS and has greatly improved support for samd21-based boards.
It currently still depends on Go 1.11, support for Go 1.12 will be added in a future release.

• compiler
  • remove old -initinterp flag
  • add support for macOS
• cgo
  • add support for bool/float/complex types
• standard library
  • device/arm: add support to disable/enable hardware interrupts
  • machine: add CPU frequency for nrf-based boards
  • syscall: add support for darwin/amd64
• targets
  • circuitplay_express: add support for this board
  • microbit: add regular pin constants
  • samd21: fix time function for goroutine support
  • samd21: add support for USB-CDC (serial over USB)
  • samd21: add support for pins in port B
  • samd21: add support for pullup and pulldown pins
  • wasm: add support for Safari in example