Broad, sweeping improvements

[sarahsturgeon]

This PR ballooned up with a few different fixes and enhancements.

Currently, it:

• Adds retries for unsuccessful failures
• Utilizes the Range header (supported in: Add Range header support gm_express_service#9) to download in chunks
• Handles all HTTP errors properly
• Removes send delay, due to Add R2 fallback for strange 404s gm_express_service#10 making it unnecessary.
• Allows operators to easily overwrite the HTTP library that Express uses
• Adds additional validation on function calls to help users easily fix mistakes
• Adds the ability to express.Send raw string data, skipping the serialization step
• Adds the small message fallback - very small messages will use NetStream instead of express (addresses: Add "small message" net fallback #36)
• Replaces json/compression with SFS
• Adds express.Seed which allows you to seed data before you need to send it

This will probably be the second-to-last major change before I put more time into a V2 rewrite with the lessons learned on our first run through.

Todo:

• Fix the PreDL callbacks?
• Fix tests

Do you want to use this?

I believe it's currently stable enough to run.
It should fix a lot of the instability and odd behavior seen from Express's main branch.

You need to:

• Download/Clone this branch (feature/include-http-failures-in-retries)
• Set your express_domain convar to stg.gmod.express