Improve type-safety for request- and response-specific types

[cmbrandenburg]

CouchDB uses different types in requests and responses for representing the same entity. For example, an embedded attachment contains different fields as a request originating from the client (e.g., the follows field) than as a response originating from the server (e.g.., the digest field). Currently, our crate's API uses strong types for responses and a quasi builder pattern when constructing requests.

It may be better to use strong types for both requests and responses.

This issue tracks the design and implementation of strong types for request- and response-specific types.

Here is a list of types for consideration:

• Documents, retrieved via GET /db/doc and updated via PUT /db/doc.
• Embedded attachments, retrieved via GET /db/doc and updated via PUT /db/doc.
• Standalone attachments, retrieved via GET /db/doc/attachment and updated via PUT /db/doc/attachment.

[cmbrandenburg]

Closed as out-of-scope for this crate's new purpose, as of v0.6.0.