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.
