Practical docs for Zome Calls

[mattyg]

Must include:

• code examples
• practical, terse explanation of how to use
• gotchas

Potentially split into multiple pages

Topics to cover:

• Zome functions and calls
  • #[hdk_extern]
  • single parameter
  • return value type
  • short-circuiting fallible functions with ?
  • Remote calls
  • Cross-zome/cell calls
  • Capabilities
  • Specially supported pattern: fire-and-forget remote signals
  • post_commit
    • infallible, read-only, but can call() another function
    • Warning against sending signals in a zome call unless you know what you're doing (writes may not succeed)
  • source chain head moved errors

[pdaoust]

Broken up into #491, #492, and #493 (with the exception of signals, which I now think we should deal with in a separate article)

[pdaoust]

Closing now that it's broken into story-sized chunks.