Add a package for type-safe byte slice pooling
Add a package for type-safe byte slice pooling

Add a package that exposes safe, strongly-typed methods to interact with a pool
of `[]byte`. This is useful throughout zap, but it's also helpful for users
implementing `json.Marshaler`, `text.Marshaler`, and the like.

uber-go/zap,uber-common/zap,uber-go/zap