Add dedicated Key Type

[robb]

This adds a dedicated AttributeKey type and allows any Hashable value as an attribute value.

The idea being that this allows for safe storage of arbitrary data inside a node.

• raw keys are meant for well-defined HTML attributes.
• scoped keys should represent user-defined keys like data- or ng-.
• .ephemeral keys are for associating short-lived data with elements, e.g. to tag them while passing them through middleware. .ephemeral will never be serialized.

Closes #28

[robb]

I guess what we want for values is AnyTextOutputStreamable, there isn't really a Hashable requirement…