Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
It is always implemented exactly the same way (as an uint8) so there is
no reason to implement it in each target separately.
This also makes it easier to add some documentation to it.
Instead of trying to modify periperhals directly, external functions are
called. For example, __tinygo_gpio_set sets a GPIO pin to a specified
value (high or low). It is expected that binaries made this way will be
linked with some extra libraries that implement support for these
functions.
One particularly interesting case is this experimental board simulator:
https://github.com/aykevl/tinygo-play
Compiling code to WebAssembly with the correct build tag for a board
will enable this board to be simulated in the browser.
Atmel/Microchip based SAMD boards are not currently supported, because
their I2C/SPI support is somewhat uncommon and harder to support in the
machine API. They may require a modification to the machine API for
proper support.
