JSON pull parsing API isn't as efficient as it should be

[yorickpeterse]

Commit 0e34cc3 adds support for parsing JSON documents without the need to allocate intermediate Json values. Unfortunately this API exposes a new problem: Inko's lack of uninitialized/partially initialized memory means you have to allocate some sort of dummy value, then update that in-place or overwrite it with the parsed data. In other words, you end up with something like this:

let val = Something(
  location: Location.default,
  ...
)

try parser
  .object
  .key('location', fn { Location.parse(parser).map(fn (v) { val.location = v }) })
  .require_all

Here the initial Location is redundant, given require_all requires a value to be parsed from the input stream.

Since closures capture by value (such that they can outlive their stack frames), you can't use a bunch of local variables of type Option[Whatever] either, as the assignment isn't visible outside the closure. Even if it was, you're still creating a bunch of Option values followed by a redundant "is it in fact a Some and not a None?" check after the require_all.

We'll need something better that doesn't suffer from these issues.