Finish transition to hex_core

[wojtekmach]

We're already using hex_core in a few places, this is what is left:

• Replace Hex.API.* with :hex_api_*.
• Convert Hex.HTTP to a :hex_http adapter
• Update Hex.Repo to :hex_registry to decode data
• Update Hex.Repo to :hex_repo to fetch data
• Move Hex.Crypto* to hex_core
• Remove Hex.Crypto in favor of directly vendored pbcs

[cgerling]

@wojtekmach are any of the items still required to be done?

[ericmj]

@cgerling They are all up for grabs 👍

[cgerling]

Cool, I intend to create a PR to address the 1st, 3rd, and 4th items.

[cgerling]

I noticed that :hex_repo doesn't have some functions that Hex.Repo needs, like get_docs/3, get_public_key/1, and get_installs/0, should I add those in hex_core before updating Hex.Repo?

[ericmj]

I noticed that :hex_repo doesn't have some functions that Hex.Repo needs, like get_docs/3, get_public_key/1, and get_installs/0, should I add those in hex_core before updating Hex.Repo?

Yes. Can you open a PR to hex_core that adds the necessary functions?