internal/task: add non-atomic atomic operations

This adds some non-atomic types that have the same interface as the ones
in sync/atomic.

We currently don't need these to be atomic (because the scheduler is
entirely cooperative), but once we add support for a scheduler with
multiple threads and/or preemptive scheduling we can trivially add some
type aliases under a different build tag in the future when real
atomicity is needed for threading.

src/internal/task/atomic-cooperative.go

@@ -0,0 +1,41 @@
+package task
+
+// Atomics implementation for cooperative systems. The atomic types here aren't
+// actually atomic, they assume that accesses cannot be interrupted by a
+// different goroutine or interrupt happening at the same time.
+
+type atomicIntegerType interface {
+	uintptr | uint32 | uint64
+}
+
+type pseudoAtomic[T atomicIntegerType] struct {
+	v T
+}
+
+func (x *pseudoAtomic[T]) Add(delta T) T { x.v += delta; return x.v }
+func (x *pseudoAtomic[T]) Load() T       { return x.v }
+func (x *pseudoAtomic[T]) Store(val T)   { x.v = val }
+func (x *pseudoAtomic[T]) CompareAndSwap(old, new T) (swapped bool) {
+	if x.v != old {
+		return false
+	}
+	x.v = new
+	return true
+}
+func (x *pseudoAtomic[T]) Swap(new T) (old T) {
+	old = x.v
+	x.v = new
+	return
+}
+
+// Uintptr is an atomic uintptr when multithreading is enabled, and a plain old
+// uintptr otherwise.
+type Uintptr = pseudoAtomic[uintptr]
+
+// Uint32 is an atomic uint32 when multithreading is enabled, and a plain old
+// uint32 otherwise.
+type Uint32 = pseudoAtomic[uint32]
+
+// Uint64 is an atomic uint64 when multithreading is enabled, and a plain old
+// uint64 otherwise.
+type Uint64 = pseudoAtomic[uint64]