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/logicblocks/halboy)
    resource serialisation.
  - Adds a default handler for `:handle-exception` which logs the exception
    when a logger implementing [[cartus.core/Logger]] is present on the
    `resource` at `:logger` and responds with a HAL resource that does not
    disclose details of the exception but does give it an ID for debugging
    purposes.
  - Adds default handlers responding with an empty HAL resource for
    `:handle-not-found`, `:handle-unauthorized`, `:handle-forbidden` and
    `:handle-method-not-allowed`.
  - 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/logicblocks/halboy) resource support
will add a `:discovery` link to any returned resource and expects a
`:router` supported by [hype](https://github.com/logicblocks/hype) to be
available in the `context`, containing a route named
`:discovery`. The [[liberator.mixin.hypermedia.core|hypermedia mixin]] adds
a router to the `context` so nothing further is needed if that mixin is in
use.