First pass implementation

[Jake-Shadle]

This is a first pass implementation of the crate. It mostly does the same thing (including copied code for the metadata structures) as crates-index, with some notable differences.

• IndexCache is allows R/W access to the metadata cache entries located in registry/index/<dir>/.cache. All methods are public, and is used by the registry specific implementations.
• SparseIndex and GitIndex build on top of IndexCache with registry specific functionality. Notably neither of them do any network I/O on their own, and thus are always available.
• git feature adds the RemoteGitIndex type which uses gix to allow both remote fetch/clone operations for a local index, as well as the ability to read git blobs to populate the local .cache
  • For 0.20.0: Make git2 dependency optional via new "git-index" feature frewsxcv/rust-crates-index#107
  • Replace git2 with gitoxide frewsxcv/rust-crates-index#82
• sparse feature adds the Async/RemoteSparseIndex types which are backed by reqwest for making requests and handling responses for remote HTTP sparse index servers
• The functionality for determining the local paths for a registry index the same way that cargo does is publicly exposed so that end users can use it without potentially needing to duplicate the same logic.
  • Massive refactor to use tame-index/gix instead cargo-fetcher#180