Added ~ alias to all #value getter methods.

@mighe @chrisseaton @lucasallan @pitr-ch I saw this alias used in [another concurrency gem](https://github.com/meh/ruby-thread). I know it's only syntactic sugar, but I liked that it enabled this syntax:

```ruby
require 'concurrent'

x = Concurrent::AtomicFixnum.new(42)
puts ~x
```

We regularly use the `<<` method an an alias for `post`, so there is synergy.

What do you think about this PR? If everyone likes this idiom I'll add a test for each of the updated classes. I don't feel strongly about it so I don't mind deleting the PR if others don't like it.