Separate files for each atomic type

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.

bool.go

@@ -0,0 +1,100 @@
+// Copyright (c) 2020 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package atomic
+
+import (
+	"encoding/json"
+	"strconv"
+	"sync/atomic"
+)
+
+// Bool is an atomic Boolean.
+type Bool struct {
+	nocmp // disallow non-atomic comparison
+
+	v uint32
+}
+
+// NewBool creates a Bool.
+func NewBool(initial bool) *Bool {
+	return &Bool{v: boolToInt(initial)}
+}
+
+// Load atomically loads the Boolean.
+func (b *Bool) Load() bool {
+	return truthy(atomic.LoadUint32(&b.v))
+}
+
+// CAS is an atomic compare-and-swap.
+func (b *Bool) CAS(old, new bool) bool {
+	return atomic.CompareAndSwapUint32(&b.v, boolToInt(old), boolToInt(new))
+}
+
+// Store atomically stores the passed value.
+func (b *Bool) Store(new bool) {
+	atomic.StoreUint32(&b.v, boolToInt(new))
+}
+
+// Swap sets the given value and returns the previous value.
+func (b *Bool) Swap(new bool) bool {
+	return truthy(atomic.SwapUint32(&b.v, boolToInt(new)))
+}
+
+// Toggle atomically negates the Boolean and returns the previous value.
+func (b *Bool) Toggle() bool {
+	for {
+		old := b.Load()
+		if b.CAS(old, !old) {
+			return old
+		}
+	}
+}
+
+func truthy(n uint32) bool {
+	return n == 1
+}
+
+func boolToInt(b bool) uint32 {
+	if b {
+		return 1
+	}
+	return 0
+}
+
+// MarshalJSON encodes the wrapped bool into JSON.
+func (b *Bool) MarshalJSON() ([]byte, error) {
+	return json.Marshal(b.Load())
+}
+
+// UnmarshalJSON decodes JSON into the wrapped bool.
+func (b *Bool) UnmarshalJSON(t []byte) error {
+	var v bool
+	if err := json.Unmarshal(t, &v); err != nil {
+		return err
+	}
+	b.Store(v)
+	return nil
+}
+
+// String encodes the wrapped value as a string.
+func (b *Bool) String() string {
+	return strconv.FormatBool(b.Load())
+}