Term and LeafAnswer serde serialization

[bakaq]

As a lot of things moved around, I thought making a new PR would be easier than rebasing #2493. This also gives us a clean slate to discuss. The old PR was getting a bit too big, and Github is notoriously bad for big threads of discussions in issues/PRs.

This basically gets to the same point that #2493 was, but with a change to rationals following #2505 (comment) (basically now they are serialized as {"rational": {"numerator": ..., "denominator": ...}}), and also without migrating the integration tests. I believe this will still change a bunch so I defer migrating the integration tests (which depend on serialization) to after this gets merged. This also doesn't have the deserialization from #2505.

@jjtolton, if you really want to continue the JSON API route in #2465 you should rebase onto this. I feel maybe just doing C APIs for dealing with Term (and with that wrapping libraries can do JSON themselves in whatever way they want) will be a faster path to merging than waiting to reach consensus on the serialization format here.

Most relevant points in the discussion until now, to recap:

• Serialization formats list: Value and QueryResolution serialization with serde #2493 (comment)
• It seems we mostly agreed to using JSON types where it makes sense, and internal tagging for everything else: Value and QueryResolution serialization with serde #2493 (comment). This is not how it's implemented now in this PR, but this would be quite easy to migrate to.
• Because JSON is kinda weird with numbers (often implementations just use f64 for all numbers), maybe it makes sense to always encode numbers as strings, tough it seems kinda unfortunate to force users to parse numbers even for small integers: Value and QueryResolution serialization with serde #2493 (comment) Value and QueryResolution serialization with serde #2493 (comment)

Also, tip when writing JSON on Github: use json5 in codeblocks. It allows things like comments without freaking out. Compare:

// json
{ "a": 1 }

// json5
{ "a": 1 }