Package-level declarations

Defines the notion of a *Render Context* for server-side pages in Gust. This protocol message is filled out with data
as configured by the developer (from `application.yml` and the active controller, as applicable) and provided as an
injected value to all server-side render operations.

When preparing context in Gust, adding a static script or stylesheet is done through `Context`. Setting page metadata
like the title, keywords, and manifest - also all done through `Context`. This of course has the added benefit of
being completely serializable, repeatable, and so on (i.e. all the benefits Protobuf brings).

Major context modules include:
- *Metadata*: Page-level metadata.
- *OpenGraph*: Properties for OG tags.
- *Twitter*: Properties for Twitter cards.
- *AppManifest*: Information to link or derive a web application manifest.
- *Styles*: Stylesheets and inline head-injected styles ("lifted styles").
- *Scripts*: Scripts to include, and inline head-injected logic ("lifted JS").

Enumerates semantic metadata formats that are supported for rendering into the page.

Enumerates known semantic types, which are further mapped to their implementation types in each
semantic expression framework by implementing code.

Defines the structure of attached content media, which is intended for emission in a given web page
via some sort of render process, depending on the context in which this media is used.

Defines broad types of media that may be expressed.

Defines the structure of top-level metadata with regard to semantic content, attached to a given
web page for emission in meta tags in various formats.

Enumerates known or supported video asset providers.

Describes resolutions at which video assets may be made available.