Rather than hand-writing the atomic integers, generate them
automatically from the same template. The generator is at
internal/gen-atomicint.
To ensure these are never out of date, add another `make` target which
verifies that the working tree is left unchanged after regenerating
code.
In anticipation of automatically generating these definitions, pull the
definitions of the various integer types into their own files as well as
their tests.
This will make it easier to review the changes when each of these files
is generated independently and automatically.