Rust: Add `enrich` API and initial set of enrichment sources

[nokome]

We currently have the notion of "methods" which can be applied to document nodes (e.g a CodeChunk, a Organization). These methods include encode, compile, execute. This issue is to add a new method, enrich that will add, and potentially replace properties of a node using third party APIs, including:

• ORCID for Person: See https://github.com/stencila/encoda/tree/master/src/codecs/orcid
• Crossref for CreativeWork: See https://github.com/stencila/encoda/tree/master/src/codecs/crossref
• GitHub for SoftwareSourceCode