Zstd support over Streams

[fairingrey]

Description

Adds support for compressing/decompressing Zstd over std futures' streams.

Motivation and Context

Helps contribute to #12. While Zstd isn't exactly well-known in the HTTP compression space, adding support for it would be nice for anyone who wants to use it, as it is comparable to (and at times, faster than) Brotli.

How Has This Been Tested?

Tests are provided!

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[fairingrey]

r? @Nemo157

You can do it after #14 is merged, since I rebased all my commits here on top of yours 😄

[fairingrey]

Actually, I still might probably change the API by including which encoding level to pick, similar to Brotli, but other than that I think it should be good to go.

EDIT: Done

[Nemo157]

LGTM other than that one comment 👍