staticcheck: Fix unused fields

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.

float64.go

@@ -29,7 +29,7 @@ import (
 
 // Float64 is an atomic type-safe wrapper for float64 values.
 type Float64 struct {
-	nocmp // disallow non-atomic comparison
+	_ nocmp // disallow non-atomic comparison
 
 	v Uint64
 }