t-rust: Design and implement the indexing for Rust crates in PurlDB

[pombredanne]

Enhanced code indexing for Rust to crawl crates.io and index all crates or a select subset of crates. Determine and index the corresponding source code repositories

[chinyeungli]

Relevant reference pages:
https://index.crates.io/
https://github.com/rust-lang/crates.io-index

[chinyeungli]

PR created: #820