Reorganize the Canonical Built-ins section.

[sunfishcode]

Reorganize the "Canonical Built-ins" section of Explainer.md.

• Don't quote the canon ebpf here, as it doesn't have a lot of explanatory information.
• Put builtins in their own subsections.
• Give builtins synopsis tables that include their conceptual signatures, binary encoding immediates, and canonical ABI signatures.
• Revise documentation to separate the conceptual signatures and semantics from the Canonical ABI representations.

I started out with a table in Async.md, but as I worked it ended up seeming to make sense to just evolve the content in Explainer.md. I'm curious what you think!