Support single-value cache block synax

[aprescott]

Sometimes you don't need a full key-value store, you just want to cache a single value. It would be nice to have a simple way to do that.

@cached_value ||= TimeoutCache.value do
  "foo"
end

The only way to do that with the existing syntax is:

@cache ||= TimeoutCache.new

value = @cache[:foo]
return value if value

@cache[:foo] = expensive_operation

The ||= part might be misleading. If there's a way to uniquely identify the caller, it would be cool to instead have:

def foo
  TimeoutCache.value do
    expensive_operation
  end
end

Which would, I suppose, require a class instance of TimeoutCache.