Liberator mixin to authorise a request based on an access token

Liberator mixins to add attributes to the context map for all requests.

Functions for defining liberator mixins: partial liberator resource
definitions that can be composed together to build up a liberator resource
with canned functionality.

The most important function in this namespace is [[build-resource]] which
accepts a sequence of liberator mixins (or resource definition maps) and
produces a liberator resource.

A liberator mixin to add
[HAL](https://tools.ietf.org/html/draft-kelly-json-hal-00) support to
liberator.

In short:

  - Adds `application/hal+json` as a supported media type.
  - Adds support for JSON serialisation for maps and seqs for the HAL
    media type.
  - Adds support for [halboy](https://github.com/jimmythompson/halboy)
    resource serialisation.
  - Adds a default handler responding with an empty HAL resource for
    `:handle-not-found`.
  - Adds a HAL error representation for use with the
    [[liberator-mixin.validation.core|validation mixin]].

Depends on:

  - the [[liberator-mixin.json.core|JSON mixin]],
  - the [[liberator-mixin.hypermedia.core|hypermedia mixin]].

Optionally extends:

  - the [[liberator-mixin.validation.core|validation mixin]].

### JSON serialisation support

The JSON serialisation support for the `application/hal+json` media type uses
the same underlying JSON encoder as the
[[liberator-mixin.json.core|JSON mixin]]. If that mixin is configured with a
custom JSON encoder, all `application/hal+json` responses will use the custom
JSON encoder.

### halboy `Resource` support

The [halboy](https://github.com/jimmythompson/halboy) resource support
will add a `:discovery` link to any returned resource and expects `bidi`
`:routes` to be available in the `context`, containing a route named
`:discovery`. The [[liberator-mixin.hypermedia.core|hypermedia mixin]] adds
routes to the `context` so nothing further is needed if that mixin is in use.