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.
This means that machine.UART0, machine.UART1, etc are of type
*machine.UART, not machine.UART. This makes them easier to pass around
and avoids surprises when they are passed around by value while they
should be passed around by reference.
There is a small code size impact in some cases, but it is relatively
minor.
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.
This makes it possible to assign I2C objects (machine.I2C0,
machine.I2C1, etc.) without needing to take a pointer.
This is important especially in the future when I2C may be driven using
DMA and the machine.I2C type needs to store some state.
Let's use the same default frequency everywhere, for consistency.
It could be any frequency, but 4MHz is already used for other chips and
it seems like a reasonable frequency to me (not too fast for most chips
but still reasonably fast). Oh, and 4MHz is slow enough that it can be
inspected by a Saleae Logic 4 (that sadly has been discontinued).
