Gateway Extraction & Support

[willscott]

Remaining work:

• proper prototypeChooser usage
• sizes in directory listings with unixfs ADL
• figuring out the plan for IPNS / dnslink
• support proper completion of loading requests
• Making sure all the requests within an http request end up in the same 'session'
• Port over tests.
• Decide on future landing point of gateway code (could we get support for an extracted module of gateway as the canonical go-ipfs instance?)

Help would be appreciated at this point in reviews of:

• the API of what's needed to serve the gateway