Use unified currency API endpoint

[tiffany352]

Currently Rink separately downloads and parses BTC stats as json and ECB stats as xml.

This change replaces that with a unified endpoint that sends definitions directly in a format Rink understands, which means that there is no need for an XML parser in Rink anymore.

• Adds Serialize + Deserialize to DefEntry. This requires converting several enum variants because serde is annoying like that.
• Adds a /data/currency.json endpoint in rink-web that coalesces data from blockchain.info and ECB into DefEntry json format.
• Removes the code for BTC/ECB parsing from rink-core.
• Uses the new rink-web endpoint from the CLI.

Pros

• Fewer dependencies in rink-core.
• Fewer requests made by Rink CLI.
• Fewer requests made against these third party APIs (as now only rink-web instances will, and as far as I know there is only 1).
• No longer directly hitting third party APIs.
• If blockchain.info ceases to exist like BTC-e did then I can fix it in rink-web and old versions of Rink will continue working.
• Easier to add more frequently-updating data sources in the future.

Cons

• I have to maintain https://rinkcalc.app/data/currency.json indefinitely. (I guess it's time for a status page and email alerts.)
• Adds more code in the path that could possibly fail.
• Data processing code is written in Typescript now.

Bonus

• price of bitcoin, hashrate of bitcoin, total of bitcoin properties now exist.