Generate all atomics automatically (#77)

`gen-valuewrapper` is specialized to generating type-safe atomic
wrappers around `atomic.Value`. This limitation is unnecessary. Given
functions to convert an exposed type to the underlying type (referred to
as "packing" here) and back ("unpacking"), we can generate wrappers
around any backing atomic type.

This generalizes `valuewrapper` into an `atomicwrapper` implementing
said functionality, and adding opt-in support for generating CAS, Swap,
and JSON methods.

With this, we can automatically generate bool, float64, and
time.Duration atomic wrappers on top of Uint32, Uint64, and Int64
respectively, as well as their core functionality.

Makefile

@@ -3,14 +3,14 @@ export GOBIN ?= $(shell pwd)/bin
 
 GOLINT = $(GOBIN)/golint
 GEN_ATOMICINT = $(GOBIN)/gen-atomicint
-GEN_VALUEWRAPPER = $(GOBIN)/gen-valuewrapper
+GEN_ATOMICWRAPPER = $(GOBIN)/gen-atomicwrapper
 
 GO_FILES ?= $(shell find . '(' -path .git -o -path vendor ')' -prune -o -name '*.go' -print)
 
 # Also update ignore section in .codecov.yml.
 COVER_IGNORE_PKGS = \
 	go.uber.org/atomic/internal/gen-atomicint \
-	go.uber.org/atomic/internal/gen-valuewrapper
+	go.uber.org/atomic/internal/gen-atomicwrapper
 
 .PHONY: build
 build:
@@ -29,8 +29,8 @@ gofmt:
 $(GOLINT):
 	go install golang.org/x/lint/golint
 
-$(GEN_VALUEWRAPPER): $(wildcard ./internal/gen-valuewrapper/*)
-	go build -o $@ ./internal/gen-valuewrapper
+$(GEN_ATOMICWRAPPER): $(wildcard ./internal/gen-atomicwrapper/*)
+	go build -o $@ ./internal/gen-atomicwrapper
 
 $(GEN_ATOMICINT): $(wildcard ./internal/gen-atomicint/*)
 	go build -o $@ ./internal/gen-atomicint
@@ -54,7 +54,7 @@ cover:
 	go tool cover -html=cover.out -o cover.html
 
 .PHONY: generate
-generate: $(GEN_ATOMICINT) $(GEN_VALUEWRAPPER)
+generate: $(GEN_ATOMICINT) $(GEN_ATOMICWRAPPER)
 	go generate ./...
 
 .PHONY: generatenodirty