The embedded `nocmp` field and other similar fields are unused in tests
and the rest of our code.
We can get the effect of `nocmp` without embedding the fields by using
`_ nocmp` as a field.
Generate atomic.Float64 with gen-valuewrapper by wrapping atomic.Uint64,
using math.Float64bits and math.Float64frombits to pack and unpack
float64 to uint64.
As in #73, separate each atomic type into its own file to ease review of
transition to generated code.
After moving every atomic to its own file, the atomic.go file serves
only as documentation, so rename it to doc.go.