Add HTTP support to SparseIndex

[Jake-Shadle]

• Disable rustfmt
• Add CRATES_IO_HTTP_INDEX
• Cleanup index initialization
• Add HTTP support to SparseIndex

This adds support for creating HTTP requests and parsing HTTP responses for an HTTP sparse index via the http crate. This method follows the sans-io approach to allow the calling crate to decide what HTTP client it wants to use to send HTTP requests so that this crate is not tied to any particular HTTP client implementation, nor does it have to care if the user wants async or not I/O or whatever else.

This is behind the sparse-http feature gate but that can be removed if the feature is deemed acceptable by default since it is fairly lightweight, especially when compared to git.

This implementation leaves it up to the user to decompress gzip encoded server responses, but gzip decompression would be easy to add either behind an additional feature flag, or as part of the sparse-http feature.

Resolves: #87

[Jake-Shadle]

Note this removes the separator parameter from crates_prefix in favor of unconditionally using / it is entirely pointless for how this crate is used as Windows/libgit has no issues understanding / or \ as equivalent path separators.

[kornelski]

Have you tested this on Windows?

I previously got bug reports about Windows actually caring about path separator char:
#93

[Jake-Shadle]

Ehh I guess I'll revert that, that's annoying.

[kornelski]

Thank you