Switch from XHR to Fetch API

[bduffany]

This is in preparation for switching the timing profile to use a streaming HTTP request for loading and parsing the JSON profile. XHR supports streaming but it's very ugly.

• Switch from XHR to Fetch
• Remove storedEncoding as a method param (since it's pretty niche), and instead pass the stored-encoding header directly via the standard RequestInit object
• Improve type safety of fetch() method - instead of Promise<any>, returns Promise<ArrayBuffer> when setting request type to arraybuffer, Promise<string> when setting request type to "text", etc.

Related issues: N/A

[bduffany]

(Will wait to submit this until after tomorrow's release since it's touching low-level RPC code)

[siggisim]

Maybe this will make it easier for us to one day support streaming protolet RPCs 🤞