Takahē HTTP Client

[AstraLuma]

Adds core.httpy, which is a wrapper around HTTPX, to provide the features:

• Block requests to some IP ranges (#402)
• Cleaner signature handling (just pass the HTTP client the actor you're using)

Fixes #402

[AstraLuma]

I can also make HTTP/2 default, if there aren't any concerns about that. (HTTPX does some vague self-FUD in the docs.)

[AstraLuma]

@andrewgodwin before I go and Convert All The Things, can I get a design review? I don't want to have to change 50 calls later.